Copernicus was one of the first of the renaissance thinkers to revolt against the now dogmatically accepted ideas of Aristotle. To understand why Copernicus was compelled to do this, some background information is required on the Aristotlean mindset.

Aristotelian Matter and Motion

According to Aristotle, all material things on earth were made of the four "elements": water, air, fire and earth. These elements made up the "sublunary" world. (Below the moon.) For example, a man was mostly earth (since earth was heavy) but also clearly made up partly of water (for the bodily liquids) air (since we needed to breath) and fire, which provided the heat of our bodies and most likely fired our muscles and brain.

Space above the moon had to be different, because this was the realm of the gods, god, or the prime mover (pick your own delusion.) Here, all was made up of Quintessence, which is perfect, impermeable, unchangable, indestructable, etc. It had to be, because it was perect. The sixth and last element was the divine, which made up the celestial world of God's domain, behind the wall of stars. We need not concern ourselves with this element.

Motion

On earth, the natural state of matter is rest. Motion as a consequence, is always either violent and unnatural - such as a rock being tossed by a catupult into the sky, or it is a natural correction of a previous state of imbalance - such as the rock falling back towards the earth, its natural place of rest. Once that rest is achieved, the motion stops. Another good example would be water running down to a river, where it belongs, or fire's flames pointing up to the sky, as fire is the lightest of all elements. (Additional note: Since earth usually went downward, and spirit went upwards, this explained why the early christians tended to believe that their bodies were evil - because they were made of the earth and intended to go down, while spirit was 'restrained by the body" and meant to go upwards.)

No matter the reason for movement, all movement on earth was linear - a straight line. This was imperfect movement, made necessary, because it occured on earth. In the heavens, however, natural movement was circular - the movement of all perfect heavenly bodies, that were made of quintessence. Why was it this way? Because, the christians would later "argue" it was "god's will that they do so." (This of course is not an explanation at all, however, lets continue...)

The questions begin...

I used the catapult in the earlier example, because it was devices such as catapults that lead to the questioning of Aristotle. If a rock projected from a catapult, once it was free of the catapult, was only moving due to its natural corrective force, then why did it keep moving once it left the catapult - i.e. why didn't it move straight down towards the ground. Clearly, Aristotle was wrong as he could be. The philosopher Jean Buridan put forth the wild notion that the catapult must impart upon the stone some sort of "impetus" This would lay the groundwork for a revision of motion that would not end til Newton obliterated every bit of it. But for now, all that is important to remember is that in the times before Copernicus and Newton, people believed in Aristotle. And, according to Aristotle, the heavens were quite a differnt place from the earth - a place of perfection. And the lowly earth a place of sin, corruption and degredation (yet, at the center - what an odd mix of hubris and servility.) Clearly, there was no way that the earth was out there, spinning among the stars, and any theory that made sense had to be one where the earth was at the center of the solar system.

Now, let's examine celestial models that existed previous to the works of Copernicus to understand further why Copernicus put forth his theory.

The first scientific model - a sun centered solar system

I can sense that some might take the above as a typo. It isn't. A sun-centered Solar System had been proposed as early as about 200 B.C. by Aristarchus of Samos, and it stands as the first "scientific" view on the matter. However, it did not survive under the weight of "common sense". For example, many would question Aristarchus' view thusly:

If the Earth actually spun on an axis (as required in a heliocentric system to explain the diurnal motion of the sky), why didn't objects fly off the spinning Earth?

If the Earth was in motion around the sun, why didn't it leave behind the birds flying in the air?

If the Earth were actually on an orbit around the sun, why wasn't a parallax effect observed? That is, stars should appear to change their position with the respect to the other background stars as the Earth moved about its orbit, because of viewing them from a different perspective (just as viewing an object first with one eye, and then the other, causes the apparent position of the object to change with respect to the background).

Clearly, Aristarchus was a fool, and the idea of the earth revolving around the sun was dropped. It was clear that future attempts would have to incorporate the wisdom of Aristotle.

(Seriously, the answers to the above question are as follows: The first two objections were not valid because they represent an inadequate understanding of the physics of motion that would only be corrected in the 17th century. The third objection is valid, but failed to account for what we now know to be the enormous distances to the stars. The parallax of stars was not measured conclusively until the year 1838. Thus, the heliocentric idea of Aristarchus was quickly forgotten and Western thought stagnated for almost 2000 years as it waited for Copernicus to revive the heliocentric theory.)

Ok, onward...

Celestial Models by Ptolemy and Copernicus

4 centuries later, in the 2nd century AD, Claudius Ptolemy proposed a model of the universe with the earth at the center (geocentric) on which Biblical scripture was based.. His model (shown above) depicts the earth as stationary with the sun moving around it in small, circular orbits called epicycles.

Why Epicycles?

The epicycles were made necessary by conflicting observations that showed that the planets did not move in circles. But, they MUST move in perfect circles, went the dogma, so Ptolemy discovered that if one imagined that there was an ideal point that circled the earth perfectly, and that the planet in question circled this ideal point just as the point AND the planet circled the earth, then the retrograde and accelerated stages of a planets orbit could be explained away, er explained.

The problem

The problem was that this explanation was convuluted and ugly - the ugly facet especially bothered christian theorists who felt that the heavens must be perfect - which implied beauty. The truth was that their paradigmatic and egocentrically biased needs caused them to retain a flawed theory through ad hocism (changing reality to fit a theory) when in fact they should have just disbanded the theory. But to do so was to say the heavens, god's abode, was not perfect. Heresy! Therefore, due to their own childish needs and biases they were tethered to a bad theory.

Along comes Copernicus.

Nicolaus Copernicus was every bit the fabled Renaissance man - an expert on medicine, law, mathematics and astronomy. Copernicus re-developed the Heliocentric model with the sun was at the center instead of the earth. The new model was rejected by the church, but it gradually gained popular acceptance because it provided better explanations for observed phenomena. Ironically, Copernicus' initial measurements were no more accurate than Ptolemy's, they just made more sense, an idea foreign to church Dogma. So, what drove Copernicus to make the change?

Anomolies

Anomolies are observations that contradict or are in discordance with the predictions of a theory. For example, if a theory of the movement of celestial bodies states that a planet should move in a cicular orbit within its crystalline sphere, than an observation that shows them not to move in a circular orbit should refute the theory. Anomolies indicate that a current theory is flawed, if not wholly incorrect. While they can be dealt with in an ad hoc fashion, as performed by Ptolemy and the later christian philosophers, the proper manner in dealing with them is to question the previous assumptions that made up the outmoded theory to see where they are in error. The next step is to take this new understanding and make it the basis to find a newer (Better) theory that can incorporate the anomolies as well as the theory supporting observations.

The old theory had so many outliers that it takes time to list them all. For example, many planets were soon found to require two epicycles to explain their "herky jerky" motion. But, the heavenly bodies were said to move on crystaline spheres, which were made of quintessence. Remember that quintessence was said to be impermeable? Well, how could two mutually exclusive spheres intersect if they were both impermeable? The answer is that they couldn't.

Then, there was the problem of the stars, which were held to be so far away that they were outside the orbit of Saturn, yet they circled the earth each night. If they were so far away, in order to circle the world each night, they would all have to move unimaginably fast. Again, the solution to these problems was first sought through ad hocism - God is great, and he can do anything. But why would even God work so hard when he didn't have to? Why would any intelligent god create a system that was more complicated than it had to be?

Copernicus and Ockham's razor

Copernicus, using the kind of thinking made famous by Ockham, discovered that if the earth circled the sun, it eliminated the need for half of the epicyles in Ptolmey's system. The retrograde movement of the planets would simply be when they lagged behind the earth's own rotation around the sun. And the stars would no longer need to race around the globe each night. Everything could be understood much more simply, and yet, the theory explained what was happening just as well as the flawed geocentric model.

Copernicus and the Need for Epicycles

There is a common misconception that the Copernican model did away with the need for all of the epicycles. This is not true, because Copernicus was able to rid himself of the long-held notion that the Earth was the center of the Solar system, but he did not question the assumption of uniform circular motion. Thus, in the Copernican model the Sun was at the center, but the planets still executed uniform circular motion about it. As it was learend only much later, the orbits of the planets are not circles, they are actually ellipses. As a consequence, the Copernican model, with it assumption of uniform circular motion, still could not explain all the details of planetary motion on the celestial sphere without epicycles. The difference was that the Copernican system required many fewer epicycles than the Ptolemaic system because it moved the Sun to the center.

What Copernicus' Model DID solve.

Three incorrect ideas held back the development of modern astronomy from the time of Aristotle until the 16th and 17th centuries: (1) the assumption that the Earth was the center of the Universe, (2) the assumption of uniform circular motion in the heavens, and (3) the assumption that objects in the heavens were made from a perfect, unchanging substance not found on the Earth. Copernicus challenged assumption 1, but not assumption 2. We may also note that the Copernican model implicitly questions the third tenet that the objects in the sky were made of special unchanging stuff. Since the Earth is just another planet, there will eventually be a natural progression to the idea that the planets are made from the same stuff that we find on the Earth.

Retrograde Motion and Varying Brightness of the Planets

The Copernican system by banishing the idea that the Earth was the center of the Solar System, immediately led to a simple explanation of both the varying brightness of the planets and retrograde motion: The planets in such a system naturally vary in brightness because they are not always the same distance from the Earth. The retrograde motion could be explained in terms of geometry and a faster motion for planets with smaller orbits.

Further Support for Copernicus

After witnessing himself what his idea did for astronomy, Copernicus searched through ancient Greek texts and found that at least one ancient philsopher, Aristarchus of Samos (ca 310 - 264 BC), had postulated a heliocentric system. Erastosthenes may also have supported such a view. In fact, it may well be that this was the leading view of the solar system until "Christianity took over and thanks to the enlightened and extremely tolerant views of the catholic church, the world returned to being flat and anyone daring to argue otherwise was simply put to death." (The previous was a quote from a nice web page History of Astronomy)

The existence of this previous extrinsic support motivated Copernicus to write his famous book. But he held the book from publishment until on his deathbed out of fear that Christians might well lovingly murder him for revealing the truth. Martin Luther, hearing of Copernicus' work in 1593 (four years before it was published_ had this to say:

"People gave ear to an upstart astrologer who strove to show that the earth revolves, not the heavens or the firmament, the sun and the moon.... This fool wishes to reverse the entire science of astronomy; but sacred scripture tells us that Joshua commanded the sun to stand still, and not the earth. - Martin Luther, referring to Joshua 10:13, in his series of "Table Talks" (1539)

Perhaps there will be babblers who claim to be judges of astronomy although completely ignorant of the subject and, badly distorting some passage of Scripture to their purpose, will dare to find fault with my undertaking and censure it. I disregard them even to the extent of despising their criticism as unfounded. For it is not unknown that Lactantius, otherwise an illustrious writer but hardly an astronomer, speaks quite childishly about the earth's shape, when he mocks those who declared that the earth has the form of a globe. Hence scholars need not be surprised if any such persons will likewise ridicule me.
Nicolaus Copernicus, in his Preface to De Revolutionibus Orbium Coelestium

The Christian Response To Copernicus

Following Copernicus' death, and the subsequent recognition of the implications of his work, the churchmen, both Protestant and Catholic, responded to Copernicus precisely as he feared they would. They called his work blasphemy, and argued that if the world really did circle the sun, then the bible must be false.

From Positive Atheism.com

Bible Passages Used Against Copernicus
- compilation by Cliff Walker

Bible quotes used to support Geocentricism:

Regardless of what modern Christians and Jews tell us that these passages "really" mean, these are the passages that were used in attempts to discredit Nicolaus Copernicus.

- Melanchthon, cited Ecclastes 1:4-5:
"... the Earth abideth forever. The Sun also riseth, and the Sun goeth down and hastethto his place where he arose. The wind goeth toward the south and turneth about unto the north.
Ecclesiastes 1:4-6

And then made the following argument:

"The eyes are witnesses that the heavens revolve in the space of twenty-four hours. But certain men, either from the love of novelty, or to make a display of ingenuity, have concluded that the earth moves; and they maintain that neither the eighth sphere nor the sun revolves.... Now, it is a want of honesty and decency to assert such notions publicly, and the example is pernicious. It is the part of a good mind to accept the truth as revealed by God and to acquiesce in it.

John Calvin, citing Pslam 93:1, stated:

"... the world also is stablished that it cannot be moved."
Psalm 93:1

Stated: "Who will venture to place the authority of Copernicus above that of the Holy Spirit?
- John Calvin, Commentary on Genesis.

The following passages were refered to by Luther, above:

"Then Joshua spoke to the Lord, in the day that he delivered the Amorrhite in the sight of the children of Israel, and he said before them: Move not O Sun toward Gabaon, nor thou, moon, toward the valley of Ajalon."
"And the sun and moon stood still, till the people revenged themselves of their enemies. Is not this written in the book of the just? So the sun stood still in the midst of the heaven, and hasted not to go down the space of one day."
"There was not before nor after so long a day, the Lord obeying the voice of a man, and fighting for Israel."
Joshua 10:12-14

Other passages cited:

"Who laid the foundation of the Earth, that it should not be removed forever."
Psalm 104:5

"And Isaiah the prophet cried unto the Lord: and he brought the shadow ten degrees backward by which it had gone down in the dial of Ahaz."
2 Kings 20:11

The Roman Catholic Church:

"... And whereas it has also come to the knowledge of the said Congregation that the Pythagorean doctrine -- which is false and altogether opposed to the Holy Scripture -- of the motion of the Earth and the immobility of the Sun, which is also taught by Nicolaus Copernicus in De Revolutionibus Orbium Coelestium, and by Diego de Zuñiga On Job, is now being spread abroad and accepted by many... Therefore, in order that this opinion may not insinuate itself any further to the prejudice of Catholic truth, the Holy Congregation has decreed that the said Nicolaus Copernicus, De Revolutionibus Orbium, and Diego de Zuñiga, On Job, be suspended until they are corrected.
The Roman Catholic Church, from The Decree of the Roman Catholic Congregation of the Index which condemned De Revolutionibus on March 5, 1616

Special Note: Copernicus was not literally correct to say the earth revolved around the sun. The reality is that the sun and earth both revolve around each other. However, he was quite correct to note that the earth moved and that the sun was at the center of the solar system, and as this was the main thrust of his theory, its really nothing but quibbling to raise this point.