Every fact of history is established in the same way. An event occurs. Those who observe it leave some sort of testimony of the event: a drawing or photograph, a marker, a monument, a written record, etc. People in subsequent generations view that testimony and therefore believe the event occurred.
That is why we believe in Jesus. His life and teachings were documented by witnesses; primarily, the apostles. Jesus told them, “. . . and you shall be My witnesses, both in Jerusalem, and in all Judea and Samaria, and even to the remotest part of the earth” (Acts 1:8).
The apostles were witnesses of what they saw: Jesus’ good works and miracles (Acts 10:38-39). They were witnesses of what they heard (Acts 22:15). They were witnesses of Jesus’ resurrection (Acts 2:32; 3:15; 5:32), the event that He pointed to time and again as the ultimate proof of who He was. (None of the apostles claimed to have seen Jesus actually emerge from the tomb, but they ate and drank with Him after He arose [Acts 10:41].) They were even witnesses of things that Jesus revealed after His ascension back to heaven (Acts 26:16).
There is no reason to question the credibility of the apostles’ testimony. They were in a position to see what they recorded. Their testimony did not bring fame or fortune; to the contrary, they were treated “as the scum of the world, the dregs of all things” (1 Corinthians 4:13).
People today speak of “witnessing” in a loose sense of telling what they believe. When the apostles witnessed, they were telling what they had seen and heard.
Peter wrote, “For we did not follow cleverly devised tales when we made known to you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of His majesty. For when He received honor and glory from God the Father, such an utterance as this was made to Him by the Majestic Glory, ‘This is My beloved Son with whom I am well-pleased’— and we ourselves heard this utterance made from heaven when we were with Him on the holy mountain” (2 Peter 1:16-18).
John added, “What was from the beginning, what we have heard, what we have seen with our eyes, what we have looked at and touched with our hands, concerning the Word of Life—and the life was manifested, and we have seen and testify and proclaim to you the eternal life, which was with the Father and was manifested to us—what we have seen and heard we proclaim to you also, so that you too may have fellowship with us; and indeed our fellowship is with the Father, and with His Son Jesus Christ” (1 John 1:1-3).