(Latin: "imitation of Christ"1)
A late mediaeval form of pious devotion: living a life of voluntary poverty and service, in imitation of Christ.
Throughout the history of Christianity, the "imitation of Christ" has repeatedly been a slogan used to signal a "return" to the strict devotion of early Christianity. In practice, this has often implied a willingness to suffer martyrdom. Perfect sanctity was sought through the deliberate imitation of the poor and suffering Christ.
In effect, the imitatio Christi was a form of protest against the secular aspects of the Church, and it often led to the formation of Christian sects - though it could just as easily be integrated into the Church, particularly through the monastic life (by choosing ascetic monaticism over a worldly life, monks and nuns could thus seek a deeper harmony with Christ, without breaking with the mainstream Church).
In Protestant Christianity, imitatio Christi played a strong rôle, as the catchphrase for evangelical "awakening" to "true" Christianity (in Protestant understanding, the denial of the supposed "evils" of the Roman Catholic Church, with its too-worldly ways). This view of the imitatio Christi as "true" Christianity carried over into the philosophical works of Søren Kierkegaard and Dietrich Bonhoeffer.
A separate, but closely related phenomenon is the imitatio Dei ("imitation of God") , a theological term describing religious rituals which constitute the deliberate imitation of divine or sacred events from history or myth.
Note(s):
1 The term imitatio Christi derives from the title of a devotional work De imitatione Christi, generally considered to be by Thomas à Kempis.