How was the Renaissance important to the spread of humanism and realism?

Renaissance humanism was a revival in the study of classical antiquity, at first in Italy and then spreading across Western Europe in the 14th, 15th, and 16th centuries. During the period, the term humanist referred to teachers and students of the studia humanitatis—meaning the humanities including grammar, rhetoric, history, poetry, and moral philosophy. It was not until the 19th century that this began to be called humanism instead of the original humanities, and later by the retronym Renaissance humanism to distinguish it from later humanist developments. During the Renaissance period most humanists were religious, so their concern was to "purify and renew Christianity", not to do away with it. Their vision was to return ad fontes to the simplicity of the New Testament, bypassing the complexities of medieval theology. Today, by contrast, the term humanism has come to signify "a worldview which denies the existence or relevance of God, or which is committed to a purely secular outlook".