The heliocentric theory, or the notion that the earth rotates on its axis and revolves in an orbit around the sun, was not a novelty in Galileo's day. This cosmological model had been advocated by some ancient Greek philosophers, for instance Aristarchus. But up until the seventeenth century it remained the minority opinion, mainly because it was thought that we should be able to sense the earth's motion, and that if the earth were spinning then objects would be flung off it by centrifugal force. Hence the model that prevailed was geocentrism, in which a spherical earth sits at the center of the universe, surrounded by concentric crystal spheres carrying the sun, moon, planets and stars. Ptolemy elaborated this model into an accurate system for making astronomical calculations. Because his system uses only circular motions whereas we now know that planetary orbits are actually ellipses, a complex system of epicycles was needed in order to make the model agree with observations.
Over the centuries, Christian philosophers and theologians had built up an elaborate synthesis that encompassed both the ancient Aristotelian philosophical tradition and Christian theology. This synthesis reached its apex in Thomas Aquinas' Summa Theologiae, which provided a complete and self-consistent framework for understanding mankind's position with respect to God and creation. It was the ``standard model'' of its day, a ``theory of everything'' that was considered to be capable, at least in principle, of providing an explanation for everything under the sun.
But as the seventeenth century dawned, cracks were beginning to appear in this magnificent edifice. The first major attack on the Aristotelian-Thomistic synthesis was Martin Luther's challenge to the authority of the Church. Although Luther himself and most of his followers continued to accept the primacy of divine revelation and believed in the earth-centered cosmology, their advocacy of private interpretation of Scripture opened the way to the calling into question of other authorities, including Aristotle. This new attitude meshed with the newly developing scientific method, which rejected all appeals to authority and respected only experimental demonstration and reasoned argument. Galileo was in the forefront of this latter new development. He did not see his scientific investigations as having any relevance to theology, and his earliest opponents were not churchmen but his fellow academics, who rightly saw in the emerging scientific method a profound challenge to the established modes of philosophical discourse. The Aristotelian-Thomistic synthesis was all of one piece. An attack on any part of it was an attack on the whole.
It is important to realize that Galileo's challenge was not limited to astronomy. In the science of mechanics that he was developing, he also attacked many of Aristotle's positions on motion and other natural phenomena. More importantly, he and other early scientists were developing a new method of inquiry that would ultimately lead to the discarding of Aristotle's entire methodology. The Scholastic philosophers recognized that the outcome of this dispute would have an impact on professorial appointments and promotions, and the control of academic programs. Just as in our time, the university of the seventeenth century was rife with turf battles, grudge fights, and personal recriminations. Even if Galileo had not been constitutionally inclined toward this kind of polemical disputation -- and he apparently rather enjoyed it-- the debate between the established Peripatetic school and the upstart scientists would have plunged him into the thick of it. The divide between the two camps was not along clerical versus secular lines as the popular account would make it. Rather, each side had representatives of both clergy and laity. Neither did the split follow Catholic or Protestant lines.
In order to understand the Galileo affair, it is essential to grasp the world view that prevailed in his day. The scientific method as we know it today was in its very early stages of development, and its true power had not yet become apparent. A strict modern positivist view of science would have been alien both to Galileo and to his adversaries. Galileo often supported his views with arguments lacking scientific rigor by modern standards, and incorporating many Aristotelian concepts and methods. As Scholasticism had become institutionalized, pedantry and sophistry were common. It was easy for someone without mathematical training to fail to appreciate the force of a genuine scientific argument, and to confuse it with mere rhetorical trickery. In spite of Aquinas's statements that human reason is capable of arriving at the truth about natural phenomena, many thinkers of the time felt that it was hopeless in practice to establish definite conclusions about the structure of the cosmos. These men regarded the disputes in the field either as pointless and inconsequential or, if they touched too closely on theological matters, dangerous and irresponsible.