The historian Josephus wrote about Jesus and he is regarded as one of the most accurate authorities on the Roman Empire.
Christianity was not widely adopted until the 3rd century when Constantine had a dream about painting a cross on the shields of his soldiers and winning a great battle. He had his men do this and won and took this as a sign about the authenticity of the so-called ‘Followers of the Way’.
Before this it existed in small pockets around the edge of the Mediterranean in opposition to Roman edicts that had outlawed Christianity. These secret churches continued to follow and spread the original message of Jesus (The one of peace and love, not the one twisted by the new age evangelicals into a justification of hatred and violence) despite knowing that if they got caught they would be executed.
For Jesus’ followers to continue his work in this way in the face of death must mean that there must have been some truth to his claims. If Jesus had been a fraud his movement would have died out when he did.