Tatian was a Christian theologian and writer who lived during the 2nd century CE in the Roman Empire. Ancient sources consistently call him an "Assyrian," which likely means he came from Syria or Mesopotamia, though the precise meaning of this label is debated. He studied in Rome under the teacher Justin Martyr. After Justin's death around 165 CE, Tatian is said to have returned to the eastern provinces, where he reportedly founded a strict ascetic group known as the Encratites. This group rejected marriage, wine, and meat. Later orthodox Christian writers viewed his teachings as heretical.

Only one of his works survives in full: the Address to the Greeks. This text is a forceful defense of Christian thought that attacks Greek philosophy, culture, and religion, arguing for the greater antiquity and truth of what he calls "barbarian" wisdom. His most influential work, however, was the Diatessaron, a seamless narrative created by blending the four Gospels into one continuous story. It does not survive in its original form, but is known from fragments, translations, and citations. The Diatessaron became the standard gospel text in many Syriac-speaking churches for centuries before being replaced.

According to modern scholars, Tatian occupies a complex position in early Christian history. He is recognized as an important apologist whose writings illuminate the intellectual conflict between early Christianity and Greco-Roman culture. Simultaneously, his association with extreme asceticism and his foundational role for the Encratite sect led to his condemnation by the developing orthodox tradition. His Diatessaron remains his most enduring legacy, representing a major early attempt to unify the gospel story.