The development of early Christianity within the Roman Empire is one of the most studied subjects in ancient history, but it still contains genuine historical puzzles. The gap between the Jesus movement described in the earliest Paul letters and the full-blown sacramental Christianity visible by the 2nd century is one of the more dramatic transformations in intellectual history.

Paul's letters, written in the 50s AD, predate the Gospels and give us the earliest documentary evidence for Christian belief. The Jesus of Paul is primarily a cosmic figure whose death and resurrection have eschatological significance; the historical Jesus of the Gospels — the teacher who told parables and healed the sick — is present but less central. The relationship between the two portraits is a major problem in New Testament scholarship.

The Acts of the Apostles, written by the same author as the Gospel of Luke, provides a narrative of early Christian expansion that historians use cautiously. It's a narrative with a theological agenda — showing the spread of the Spirit from Jerusalem to Rome — and its historical accuracy on specific points is variable.

The persecution of Christians in the Roman Empire was inconsistent and largely local until Decius (249-251 AD) made it systematic. The earlier persecutions under Nero (64 AD) and Domitian (81-96 AD) were targeted and limited. The question of why persecution occurred at all — given Rome's general tolerance for religious diversity — is best answered by the exclusive truth claim: Christians refused the minimal religious obligations (sacrifice to the emperor's genius) that all Roman subjects were expected to perform.