A True Witness
Throughout history, many religious teachers have appeared claiming to carry a message from God or a god. The reliability of their source and of their record has usually been difficult to verify at best. They emerge from a secret meeting with an angel in a cave or in the forest with no witnesses, and declare that they have new truth to reveal.
Jesus was different. He was able to demonstrate that He was indeed a prophet of God, and to do so He had the testimony of several reliable witnesses.
John the Immerser had shown himself to be a prophet of God, and he directly preached that Jesus is the Christ (John 1:36, 5:33-35).
The work that Jesus was doing publicly, both in His preaching and teaching, and also in His miracles provided ample proof that His claims were true (John 5:36-39).
The Scriptures of the Old Testament all point to Christ in prophecy. The prophecies are numbered in the hundreds, and He fulfills each and every one. (John 5:39ff).
We each have a duty to examine the evidence and then draw the conclusions that the evidence demands and act accordingly.