Firsthand witnesses in the Bible minimize questionability by primary source material being cited in the New Testament. Also the time between the events themselves and when they were recorded was very short leaving negligible chance for error. Peter cites "we are witnesses of all the things He did .. [and] they also put Him to death by hanging Him on a cross" (Ac 10:39) whereby we were "witness of the sufferings of Christ" (1Pe 5:1). A witness testifies in court. Jesus substantiated his responsibility saying "'the very works that I do testify about Me, that the Father has sent Me'" (Jn 5:36). Furthermore, "'If I do not do the works of My Father, do not believe Me; but if I do them, though you do not believe Me, believe the works'" (10:37). They were "signs Jesus also performed in the presence of the disciples" (20:30). Peter witnessed that it was "'Jesus the Nazarene, a man attested to you by God with miracles and wonders and signs which God performed through Him in your midst'" (Ac 2:22). John "is the disciple who is testifying to these things and wrote these things" (Jn 21:24). "These have been written so that you may believe that Jesus is the Christ" (20:31) and to know "that his testimony is true" (21:24).
It was so convincing that they "convened a council, and were saying, 'What are we doing? For this man is performing many signs. If we let Him go like this, all men will believe in Him!'" (Jn 11:47-48). Even John the Baptist sent two of his disciples to Jesus asking, "'Are You the Expected One?'" (Lk 7:20). One's faith is challenged because it was the same John where "the next day he saw Jesus coming to him and said, 'Behold, the Lamb of God who takes away the sin of the world!'" (Jn 1:29). Jesus answered, "'The blind receive sight, the lame walk, the lepers are cleansed, and the deaf hear, the dead are raised up, the poor have the gospel preached to them'" (:22). "God raised Him up on the third day" (Ac 10:40-41) and he "was declared the Son of God with power by the resurrection from the dead" (Ro 1:4). It was "granted that He become visible not to all the people, but to witnesses who were chosen beforehand" (Ac 10:41). "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" (2Pe 1:16). John testifies "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" (1Jn 1:1). "And He ordered us to preach to the people, and solemnly to testify that this is the One" (Ac 10:42). "After that He appeared to more than five hundred brethren at one time, most of whom remain until now" (1Co 15:6).