I think you might be confused about what humanism is. For Renaissance humanists, "humanism" really meant "the study of classical antiquity" (Greek and Latin texts) and not "thinking that humanity is important and God isn't," even though that is how many humanists today use the word. It's more closely related to the study of the humanities than to a belief in anything in particular about humans. Many Renaissance humanists were actually monks or priests.

They did see religion in a different way than many of their medieval predecessors had, however. For example, the same impulse toward reading old texts in the original led more of them to read the bible in Greek and Hebrew, rather than relying on the Latin translation used throughout the middle ages. This made new interpretations possible, because scholars could argue that the bible did not actually mean what people thought it meant. In a less obviously-definable way, they also, by and large, had a higher opinion of people and thought they were less necessarily sinful than many medieval scholars had believed. But, with very few exceptions before Luther, they definitely still identified as Christians and believed in the church.

A few good books to look at: almost any of Grafton's books, especially Bring Out Your Dead or Worlds Made by Words; and Paul Oskar Kristeller's Renaissance Thought and its Sources.