Was Renaissance Humanism anti-Christian? Yes, if Catholicism is predicated of Christianity. Humanism seeks the encomium of man. Catholicism seeks the encomium of God. To praise both at once is inconsistent. Prior to the 15th century, the claim 'I am Christian' was the same as 'I am Catholic', but the Renaissance is the first time when Christianity may be differentiated from Catholicism. The difference was a shift in emphasis of a metaphysical God to the emphasis of God's manifestation of beauty in man and the world. The result of that shift was Humanism.
Catholicism had traditionally taught and encouraged a diminutive perspective of man with respect to God. Humans were at the mercy of divine law. St. Augustine recognized the significance of divine law. He proposed to separate the function of church and state; the domain of divine law is that of the church while the domain of worldly well being is that of the state. St. Augustine also argued that humanity requires the function of state regulation to provide the conditions under which humanity can have a fulfilled life; humans need to protect themselves from their own sin. But from the 5th century less were St. Augustine's views observed. The papacy grew in power, increasingly contentious with the state. Eventually the top echelons of the church were oppressing their people.
Many of our authors, perhaps beginning with Chaucer and culminating with Pico, wrote about ideas that rejected medieval views about the diminutive nature of man. Some early political theorists, like Machiavelli or More, argued against Catholic oppression. Near the end of the development of the Renaissance Humanism stands Descartes. He axiomaticised a philosophical system which did not depend on revelation or theological principles. He came "perilously close to saying that there is truth that conflicts with the bible."
Most Humanists were enthusiastic about Greek and Roman literary and historical traditions. Some had expressed contempt for the Italian vernacular. The language of academic expression was Latin. The scholars were expected to know Greek and Arabic also. Pico's intention to create a universal theology, including pagan customs, ran against church doctrine. The Humanist poetic production was inspired by classical models and written mostly in Latin or occasionally Greek. Giovanni stands as a remarkable poet. He assumed leadership of Naples Humanist Academy. His writings, like De Prudentia and De Fortuna, describe genuine human enigmas. Even More's Utopia was written in Latin, a book which addressed vulgar issues for their own benefit, yet they could not read the text.
So many figures in the Renaissance Humanism set out promulgating central and condemning facets of Catholicism, yet they maintained a Christian semblance. Christian morality was never dismissed. More described a civilization that possessed supreme morality and government: Utopia. Bruni attacked monastic ideals but wanted productive citizens. Even the Renaissance artists began to paint by illicit means, like the cadavers used by Leonardo to research the human body for the purpose of painting The Last Supper.
Humanist views segued into contemptuous reform. Since Humanist philosophy empowered the individual, the church, which is to say both Christianity and Catholicism, suffered many formal attacks from Luther, who is perhaps singly responsible for the schism of Christianity, under which fall the Protestants. So not only did Humanism confront Catholicism in a way that seems anti-Christianity, but it also adumbrates a grim future for the Catholicism's unity.
To say that Renaissance Humanism was wholly anti-Christian is hasty at best. Rather, there was a shift in emphasis from the papal authority, ex cathedra, to the beauty of humanity as it was established by God. The Humanist writers and artists were no longer interested in a pellucid God; they sought beauty as she appears in human nature.