A Christian missionary can be defined as "one who is to witness across cultures."^[2] The Lausanne Congress of 1974, defined the term, related to Christian mission as, "to form a viable indigenous church-planting movement." Missionaries can be found in many countries around the world.

Jesus instructed the apostles to make disciples of all nations (Matthew 28:19–20). This verse is referred to by Christian missionaries as the Great Commission and inspires missionary work.

Village of christianized Tapuyos Indians, Brazil c. 1820

The New Testament missionary outreach of the Christian church from the time of St Paul was extensive throughout the Roman Empire. During the Middle Ages the Christian monasteries and missionaries such as Saint Patrick, and Adalbert of Prague propagated learning and religion beyond the boundaries of the old Roman Empire. In the 7th century Gregory the Great sent missionaries including Augustine of Canterbury into England. During the Age of Discovery, the Roman Catholic Church established a number of Missions in the Americas and other colonies through the Augustinians, Franciscans and Dominicans in order to spread Christianity in the New World and to convert the Native Americans and other indigenous people. At the same time, missionaries such as Francis Xavier as well as other Jesuits, Augustinians, Franciscans and Dominicans were moving into Asia and the Far East. The Portuguese sent missions into Africa. These are some of the most well-known missions in history. While some of these missions were associated with imperialism and oppression, others (notably Matteo Ricci's Jesuit mission to China) were relatively peaceful and focused on integration rather than cultural imperialism.

Much contemporary Catholic missionary work has undergone profound change since the Second Vatican Council, and has become explicitly conscious of Social Justice issues and the dangers of cultural imperialism or economic exploitation disguised as religious conversion. Contemporary Christian missionaries argue that working for justice is a constitutive part of preaching the Gospel, and observe the principles of inculturation in their missionary work.

As the church normally organizes itself along territorial lines, and because they had the human and material resources, religious orders—some even specializing in it—undertook most missionary work, especially in the early phases. Over time a normalised church structure was gradually established in the mission area, often starting with special jurisdictions known as apostolic prefectures and apostolic vicariates. These developing churches eventually intended 'graduating' to regular diocesan status with a local episcopacy appointed, especially after decolonization, as the church structures often reflect the political-administrative reality.

The Eastern Orthodox Church, under the Orthodox Church of Constantinople was vigorous in its missionary outreach under the Roman Empire and continuing Byzantine Empire, and its missionary outreach had lasting effect, either founding, influencing or establishing formal relations with some 16 Orthodox national churches including the Romanian Orthodox Church, the Georgian Orthodox and Apostolic Church and the Ukrainian Orthodox Church (both said to have been founded by the missionary Apostle Andrew), the Bulgarian Orthodox Church (said to have been founded by the missionary Apostle Paul). The two 9th century saints Cyril and Methodius had extensive missionary success in Eastern Europe. The Byzantines expanded their missionary work in Ukraine after a mass baptism in Kiev in 988. The Serbian Orthodox Church had its origins in the conversion by Byzantine missionaries of the Serb tribes when they arrived in the Balkans in the 7th century. Orthodox missionaries also worked successfully among the Estonians from the 10th to the 12th centuries founding the Estonian Orthodox Church.

Jesuits who were martyred by the Araucanian Indians in Elicura in 1612

Under the Russian Empire of the 19th century, missionaries such as Nicholas Ilminsky moved into the subject lands and propagated Orthodoxy, including through Belarus, Latvia, Moldova, Finland, Estonia, Ukraine, and China. The Russian St. Nicholas of Japan took Eastern Orthodoxy to Japan in the 19th century. The Russian Orthodox Church also sent missionaries to Alaska beginning in the 18th century, including Saint Herman of Alaska, to minister to the Native Americans. The Russian Orthodox Church Outside Russia continued missionary work outside Russia after the 1917 Russian Revolution, resulting in the establishment of many new dioceses in the diaspora, from which numerous converts have been made in Eastern Europe, North America and Oceania.