Christian sources are biased by definition, and Matthew and Luke copied Mark. How can four Gospels plus Paul count as five witnesses?

Every ancient source has a viewpoint; historians do not discard Josephus because he was a Jewish partisan or Tacitus because he hated Nero. The right question is whether the bias would produce this particular claim, and a crucified messiah is the opposite of what a Jewish movement would invent. On independence, the point is fair: Matthew and Luke depend on Mark for much of the passion narrative, so the Synoptics should be counted cautiously. The weight falls on Paul, whose letters predate Mark and who had met Peter and James, alongside Mark and John. That is still more attestation than survives for almost any event of the first century.