One of the main projects of many Renaissance thinkers was to discover the similarities between Christian thinking and the thinking of the ancient Greeks and Romans. Certainly this was one of the main interests of Francesco Petrarca (known in English as Petrarch), who believed that Christianity was the Truth with a capital T and who therefore believed that anything true in the writings of the ancients was therefore automatically compatible with Christianity. This assumption that many classical writings posed no real threat to Christianity but in fact helped to buttress Christian thinking helped make classical thinkers appealing to many intellectuals during the Renaissance. If classical thinkers had been perceived as dangerous threats to Christianity, classical thinking would probably not have been embraced nearly as readily in the Renaissance as it was.
I agree with everything in the previous post, and would add that an emphasis on humanism, i.e., Greek and Roman learning, opened up new areas for intellectual inquiry. Where academics had been almost entirely concerned with religious disputation, humanists (while still quite interested in religious questions themselves) turned their attention to secular issues. This lead to the revitalization of a number of classical disciplines, including history and poetry.
Humanists were very interested in learning from the old classics. They wanted to be more like the ancient Greeks and Romans who were urban and secular just like the humanists were. So they advanced learning because they wanted to learn Greek and Latin to read the classics and then they wanted to learn from those classics once they were able to read them.