The historical phenomenon of Christianization, (or Christianisation — see spelling differences) the conversion of individuals to Christianity or the conversion of entire peoples at once, also includes the practice of converting native pagan practices and culture, pagan religious imagery, pagan sites and the pagan calendar to Christian uses, due to the Christian efforts at proselytism (evangelism) based on the tradition of the "Great Commission".

The process of Christianization has at times been relatively peaceful and at times has been a very violent process, ranging from political conversions to adopt Christianity to military campaigns to force conversion onto native populaces.

In Antiquity, Christianization was effected only partly through laws against indigenous religious practices, official conversions of temples to Christian churches and the placement of Christian churches over ancient religious sites. It was effected also by the demonization of indigenous pagan gods, traditional practices into witchcraft and the partial Christianization to outright banning of existing rites under threat of torture and death.

Reformatting native religious and cultural activities and beliefs into a Christianized form was officially sanctioned; preserved in the Venerable Bede's Historia ecclesiastica gentis Anglorum is a letter from Pope Gregory I to Mellitus, arguing that conversions were easier if people were allowed to retain the outward forms of their traditions, while claiming that the traditions were in honour of the Christian God, "to the end that, whilst some gratifications are outwardly permitted them, they may the more easily consent to the inward consolations of the grace of God". In essence, it was intended that the traditions and practices still existed, but that the reasoning behind them was forgotten. The existence of syncretism in Christian tradition has long been recognized by scholars, and in recent times many of the instances of syncretism have also been acknowledged by the Roman Catholic church.

Humanistic studies of Antiquity and the Reformation combined in the sixteenth century to produce works of scholarship marked by an agenda that was occupied with identifying Roman Catholic practices with paganism, and identifying the emerging Protestant churches with a purgative "re-Christianization" of society. The Lutheran scholar Philip Melanchthon produced his Apologia Confessionis Augustanae (1530) detailing the rites derived from pagan practices. Heinrich Bullinger, De origine erroris libris duo (1539) detailed the pagan "origins of (Catholic) errors".

Isaac Casaubon, De rebus sacris et ecclesiasticus exercitationes (1614) makes a third familiar example, where sound scholarship was somewhat compromised by sectarian pleading. Thus such pagan precedents for Christian practice have tended to be downplayed or even sometimes dismissed by Christian apologists as a form of Protestant Apologetics.

The 20th century saw more purely historical inquiries, free of sectarian bias; an early historicist classic in this field of study was Jean Seznec's The Survival of the Pagan Gods: the mythological tradition and its place in Renaissance humanism and the arts. (1972).