How do you have witnesses to the resurrection? The disciples didn’t even see Him rise from the dead. They saw Him after He had risen as it was evidenced by the wounds in His hands and His side when He appeared to them. The disciples were the ones who had spent years with Him and watched Him do miracles and watching Him heal the sick and teaching great wisdom and love and needed to see Him. After Jesus was crucified their faith in Him was destroyed. Jesus needed to appear to them to establish the truth of who He said He was, God in flesh (John 8:24).
Nonbelievers simply will not believe, thus (non-believers). However, there is writing from Josephus about Jesus. This is from his writing in AD 66 and is an extra-biblical source:
“Now there was about this time Jesus, a wise man, if it is lawful to call him a man; for he was a doer of wonderful works, a teacher of such men as receive the truth with pleasure. He drew over to him both many of the Jews and many of the Gentiles. He was [the] Christ. And when Pilate, at the suggestion of the principal men among st us, had condemned him to the cross, (9) those that loved him at the first did not forsake him; for he appeared to them alive again the third day; (10) as the divine prophets had foretold these and ten thousand other wonderful things concerning him. And the tribe of Christians, so named from him, are not extinct at this day.”
Although some questions arose about it favoring Jesus too much asserting it could be unauthenticated, but I add that that assertion has never been proven. However, the real issue of the resurrection deals with the evidence. Evidence that consists of the testimony of many people who professed that they had seen the risen Jesus.As well the same people who testified to the resurrection of Christ also gave up their economic and social security and their lives were put on the line to proclaim that Jesus had risen. They were persecuted and murdered.