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. The term is applied to more than one type of conversion. For example, it can describe the changes that follow an individual's conversion, and it can also be used to designate the conversion of previously non-Christian practices, spaces and places to Christian uses and names. Historically, the term has also 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 more than it is a full achievement even when laws and leaders support it.
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 about 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 north into western Europe. In the High and Late Middle Ages, it 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.
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 mission. 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.