A fellow named Claudius Ptolemy, a Greek astronomer who lived 1700 years before Galileo, had theorized that everything must revolve around the Earth. After all, it was easy to go outside, look up, and actually observe this! An Earth-centered (geocentric) model of the universe fit very well with the Christian Church's theories that Man was the center of all God's creations. Besides, a passage in the Bible indicated that "the Sun stood still in the sky" , at Joshua's request, to allow time to finish a particular battle. If the Sun had stood still, then the Sun must indeed orbit the Earth. The Church had long-ago accepted, and felt comfortable with, Ptolemy's theories.
Meanwhile, Nicholas Copernicus, a Polish scientist living about a century before Galileo, had already come up with the unorthodox idea that the Sun was at the center of the solar system. Galileo knew about and had accepted Copernicus's heliocentric (Sun-centered) theory.