Christianization (or Christianisation) is a term for the specific type of change that occurs when someone or something has been or is being converted to Christianity. Christianization has, for the most part, spread through missions by individual conversions, but has also been the result of violence. Christianization is also the term used to designate the conversion of previously non-Christian practices, spaces and places to Christian uses and names. In a third manner, the term has been used to describe the changes that naturally emerge in a nation when sufficient numbers of individuals convert, or when secular leaders require those changes. Christianization of a nation is an ongoing process.

It began in the Roman Empire when the early individual followers of Jesus became itinerant preachers in response to the command recorded in Matthew 28:19 (sometimes called the Great Commission) to go to all the nations of the world and preach the good news of the gospel of Jesus. Christianization spread through the Roman Empire and into its surrounding nations in its first three hundred years. The process of Christianizing the Roman Empire was never completed, and Armenia became the first nation to designate Christianity as its state religion in 301.

After 479, Christianization spread through missions north into western Europe. In the High and Late Middle Ages, Christianization was instrumental in the creation of new nations in what became Eastern Europe, and in the spread of literacy there. In the modern era, Christianization became associated with colonialism, which, in an almost equal distribution, missionaries both participated in and opposed. In the post-colonial era, it has produced dramatic growth in China as well as in many former colonial lands in much of Africa. Christianization has become a diverse, pluralist, global phenomenon of the largest religion in the world.

Historian Dana L. Robert has written that the significant role of Christianization in shaping multiple nations, cultures and societies is understandable only through the concept of Christian mission. Missionaries "go out" among those who have not heard the gospel and preach. Missions, as the primary means of Christianization, are driven by a universalist logic, cannot be equated with western colonialism, but are instead a multi-cultural, often complex, historical process.

David Abulafia and Nóra Berend speak of religious activity in relation to the "frontier" regions at the borders of civilizations. Berend sees a frontier as "a contact zone where an interchange of cultures was constantly taking place". In this way, the missionary religions of Buddhism, Islam and Christianity spread themselves geographically, through teaching and preaching, with interaction sometimes producing conflict, and other times mingling and accommodation.

Alan Neely writes that, "wherever Christianity (or any other faith) is carried from one culture to another, intentionally or not, consciously or not, it is either adapted to that culture or it becomes irrelevant." In his book Christian Mission, Neely provides multiple historical examples of adaptation, accommodation, indigenization, inculturation, autochthonization and contextualization as the means of successful Christianization through missions. Neely's definitions are these:

James P. Hanigan writes that individual conversion is the foundational experience and the central message of Christianization, adding that Christian conversion begins with an experience of being "thrown off balance" through cognitive and psychological "disequilibrium", followed by an "awakening" of consciousness and a new awareness of God. Hanigan compares it to "death and rebirth, a turning away..., a putting off of the old..., a change of mind and heart". The person responds by acknowledging and confessing personal lostness and sinfulness, and then accepting a call to holiness thus restoring balance. This initial internal conversion is only the beginning of Christianization; it is followed by practices that further the process of Christianizing the individual's lifestyle, which according to Hanigan, will include ethical changes.

While Christian theologians such as the fourth century Augustine and the ninth century Alcuin maintained that conversion must be voluntary, there are historical examples of coercion in conversion. Constantine used both law and force to eradicate the practice of sacrifice and repress heresy though not specifically to promote conversion. Theodosius also wrote laws to eliminate heresies, but made no requirement for pagans or Jews to convert to Christianity. However, the sixth century Eastern Roman emperor Justinian I and the seventh century emperor Heraclius attempted to force cultural and religious uniformity by requiring baptism of the Jews. In 612, the Visigothic King Sisebut, prompted by Heraclius, declared the obligatory conversion of all Jews in Spain. In the many new nation-states being formed in Eastern Europe of the Late Middle Ages, some kings and princes pressured their people to adopt the new religion. And in the Northern crusades, the fighting princes obtained widespread conversion through political pressure or military coercion.