The Twenty Thousand Martyrs of Nicomedia were a body of Christians who, by tradition, were burned to death inside their church in the city of Nicomedia during the persecution under the Roman emperor Maximian. The synaxarion places their death on the feast of the Nativity of Christ, and the Orthodox Church commemorates them on December 28, during the Afterfeast of the Nativity. They are kept as a single collective commemoration rather than as named individuals.
According to the synaxarion account, the emperor Maximian (reigned 284–305) had returned victorious from a military campaign and wished to offer sacrifice before the idols. When Anthimus, Bishop of Nicomedia, gathered the Christians of the city into their church for the celebration of the Nativity, the emperor learned of the assembly and ordered that dry wood be heaped around the building and set alight, so that those within would be consumed. By tradition the bishop, forewarned, hastened to baptize the catechumens, celebrated the Divine Liturgy, and gave communion to all before the fire was kindled.
The tradition relates that the dry wood was lit and that all the Christians gathered within the church were burned alive, numbering twenty thousand. Bishop Anthimus himself was preserved unharmed and was later martyred by beheading, his own commemoration falling on September 3. The martyrdom belongs to the Diocletianic persecution, the empire-wide assault on the Church that began at Nicomedia in 303 with the destruction of the city's principal church and the burning of the Scriptures.
The persecution at Nicomedia is among the best-attested of the early fourth century, recorded by the contemporary writers Eusebius of Caesarea and Lactantius. Eusebius relates that of the Christians then living in Nicomedia, many were put to death by imperial decree, some by the sword and others by fire. Modern historians generally regard the round figure of twenty thousand, which appears in the later liturgical tradition, as a symbolic or hagiographical number rather than a precise count, while the historical reality of a severe persecution at Nicomedia is not in doubt.