While Christianitys interaction with Humanism seems to have started as early as 2nd century A.D, it wasnt until the beginning of Renaissance that the two ideologies fused substantially.
The Renaissance was a period when Europe woke up to the possibilities of human achievement. During this period, the notion of separation of State and Church is yet unrealized, and hence Christian doctrine and dogma infiltrated into political and social affairs. During the fifteenth century, however, luminaries such as Giovanni Pico della Mirandola helped bring humanistic ideas into Christianity. For example, his book Oration on the Dignitiy of Man states that transcendence into the divine is the highest form of the humanist experience. That is, by following all the tenets laid down by Humanist philosophy, human beings can move closer to God.
The Church has always been selectively accommodative of scientific discoveries that are contrary to what is revealed in the Holy books. In the classic case of Galileo Galileis discovery that the earth is not the center of the universe, the Church authorities eventually conceded this fact and modified the scriptures to reflect this (although they initially opposed and condemned both the scientist and his work). In the same vein, since Humanism values scientific knowledge, especially about human evolution, and attempts to understand human nature through empirical and historical analysis, the principles of human conduct that it espouses tend to be more in tune with times. That is, the flexibility and openness afforded by Humanism, makes it a valuable ally in ascertaining moral values.
Hence, although the Papacy and other Christian authorities had often been hostile to some of the views and guiding principles offered by Humanism, the former had eventually and gradually come round to accept