Christianization or Christianisation is to make Christian; to imbue with Christian principles; to become Christian. It can apply to the conversion of an individual, a practice, a place or a whole society. It began in the Roman Empire, continued through the Middle Ages in Europe, and in the twenty-first century has spread around the globe.

Historically, there are observable stages in the process of Christianization beginning with 1) the mission period and individual conversion, followed by 2) consolidation and community building, and 3) the exchange of beliefs and sacred spaces sometimes referred to as syncretism. Having observable stages does not indicate Christianization has maintained one consistent approach to conversion throughout its long history. Different periods and places have produced a variety of methods, motives and means.

The first countries to make Christianity their state religion were Armenia, Georgia, Ethiopia and Eritrea in the fourth century. By the end of the fifth and beginning of the sixth century, the majority of the Roman Empire had been converted. Christian empire was finally created under the Eastern Emperor Justinian the first, when Ancient Christianity begins its end, and Christianity transforms into its eclectic medieval forms by the 800's.

Medieval Christianization began in Europe in the 8th and 9th centuries. A new region of Europe that later became known as Eastern Central Europe was formed, though not without some bloodshed, since throughout central and eastern Europe, Christianization and political centralization went hand in hand. The rulers of Bulgaria, Bohemia (which became Czechoslovakia), the Serbs and the Croats, along with Hungary, and Poland, voluntarily joined the Western, Latin church, sometimes pressuring their people to follow. The Christianization of the Kievan Rus', the ancestors of Belarus, Russia, and Ukraine, began in the tenth century following the path of Byzantine Christianity and becoming a true state church with state control of religion and some coercion.

The two centuries around the turn of the first millennium brought Europe's most significant Christianization of the Middle Ages. What had been two dangerous and aggressive enemies, (the Scandinavian Vikings on the northern borders, and the Hungarians in the east), voluntarily adopted Christianity and founded kingdoms that sought a place among the European states. The Northern Crusades, from 1147 to 1316, form a unique chapter in Christianization. They were not for the reclamation of lost territory, nor were they a defense against invasive Muslims; instead, they were largely political, led by local princes against their own enemies, for their own gain, and conversion by these princes was almost always a result of armed conquest. Colonialism was both supported and opposed by missionaries. Modern Christianization has become a global phenomenon.