Tacitus and Josephus are considered non Christian sources, and both directly attest to the life of Jesus.
Polycarp who was a direct disciple of John also attested to him, being Christian. No scholar that I've seen denounces the existence of Polycarp, though (only) atheists denounce his martyrdom. I bring up Polycarps existence because..... HE WAS A DIRECT DISCIPLE OF JOHN. The same John who wrote the the Gospel.
Furthermore, while the Gnostic records were fraudulent, and meant to scam the Church as well as Roman citizenry, they continually claim that Jesus was indeed a historical character.
Finally, we were presented a full written historical record of Jesus, which was written by men who directly knew him. The Church preserves this record today as the Gospels, Letters of the Ecclesia, and Revelation of Christ.