[2] In the third century Tertullian asked if it was possible for a Christian to "ask a watchword from the emperor who has already received one from God",[4] as well as noting that Jesus had taught that "he who lives by the sword shall die by the sword";[5] however he did believe that Christians should be loyal citizens and pray for their emperor.
In the fifth century, Saint Augustine took Romans 13:4[9] as a basis for each Christian to decide for themselves if they wished to join an army.
In particular, the Crusades were initiated as a necessary war to reclaim the Holy Land against the enemies of God and were supported by Pope Urban II,[14] who offered religious rewards of forgiveness of sins and glory in heaven to those who joined the conflict.
[15] When large groups of Christians broke away from the Roman Catholic church in the 16th century, many of them followed theologians such as John Calvin and Zwingli.
[20] From the start of the Reformation some groups, particularly Anabaptists, took the position of pacificism and refused to be part of politics or the military;[21] with followers being executed for this belief.
Militia members were selected by ballot, but Quakers were allowed to pay £10 (approximately £2300 in 2023) to nominate a substitute to take their place.
In the lead-up to WWI, the Quakers' Yearly Meeting stated "We must continue to offer strenuous opposition to the establishment in any part of the British Empire of a system of compulsory military training.
[25] During the war a No-Conscription Fellowship was set up and by 1916, there was a British conscience clause which allowed conscientious objectors to argue for their non-conscription in front of a tribunal.
[26] In the US, out of 2,100 people who objected to joining the war due to religious reasons, 1890 of them agreed to take on non-combative work, mainly in factories and farms.
[27] The presence of Christians on both sides led to the unofficial Christmas truces of World War I, commemorated in the film Joyeux Noël.
In the late 20th century, several Christian denominations preached a complete ban on all combatant service, including Seventh-day Adventists,[28] Jehovah's Witnesses[29] and Christadelphians,[30] as well as Anabaptists such as Mennonites and the Amish.