Begin with the earliest sources
Paul’s letters were written within the first Christian generation. In 1 Corinthians 15:3-8 he passes on a received summary about Jesus’ death, burial, resurrection, and appearances. Galatians 1:18-19 names Peter and James, “the Lord’s brother.” These are Christian sources, but being committed does not make a source historically irrelevant. Historians ask when it was written, what it claims, and how it relates to other evidence.
The four Gospels provide extended accounts. Luke 1:1-4 explicitly describes gathering traditions and arranging an account. They differ in emphasis and detail while agreeing on a first-century Jewish teacher named Jesus who was crucified under Pontius Pilate and whose followers claimed He rose.
Non-Christian references
Josephus’ Antiquities 20.9.1 refers to James as the brother of Jesus “who was called Christ.” A longer passage in Book 18 has a disputed textual history, so it should not be quoted as uncomplicated proof. Tacitus, writing around AD 116, says Christus was executed under Pontius Pilate during Tiberius’ reign in Annals 15.44. Pliny’s letter to Trajan describes Christians singing to Christ and binding themselves to ethical conduct.
These later writers do not prove every Gospel claim. They do make the idea of an invented Jesus difficult to sustain. The larger question is what happened after His crucifixion. Read the case for the resurrection.