The New Testament was written by contemporaries, and there was of course ample opportunity for others to protest if eye-witness accounts did not work out with the current well-known facts
We can see many examples in the New Testament how the authors claim to be eye witnesses and/or knew the disciples of Jesus. This was an important criteria when it came to the canonization of Scripture. The original gospels were protected and accepted as apostolic authorship, and later texts were rejected by the early church Fathers. The earliest writings of the church fathers confirm the original gospels as being written by the persons tied to the gospels. Papias (Bishop of Hierapolis) is the most well-known example, and he for describes the gospel of Mark as a record of Peter’s teachings as an eye-witness of Jesus. Papias lived beteween 60–130 AD and excerpts of his work is known through Irenaeus of Lyon and Eusebius of Caesarea.
Matthew, John and Peter were members of the twelve original apostles, and they saw Jesus alive after His resurrection. Also James, the half-brother of Jesus, saw him after his resurrection and James was also a close associate of the apostles. Mark and Luke were contemporaries of Jesus and were also close to the apostles, as well as Jude who was another of Jesus half brothers. Though each witness will provide different perspectives, the accounts can be matched and synchronized. Many of the Bible’s authors were willing to risk their lives for the gospel, and few people would be willing to do this if they knew they defended a complete fraud.
Here are examples from the Bible about being written by eyewitnesses:
1 Cor. 15:4 And that he was buried, and that he rose again the third day according to the scriptures:5 And that he was seen of Cephas, then of the twelve:6 After that, he was seen of above five hundred brethren at once; of whom the greater part REMAIN UNTO THIS PRESENT, but some are fallen asleep. 7 After that, he was seen of James; then of all the apostles.8 And last of all he was seen of me also, as of one born out of due time.
Luke 1:1 Forasmuch as many have taken in hand to set forth in order a declaration of those things which are most surely believed among us,2 Even as they delivered them unto us, which from the beginning were EYEWITNESSES, and ministers of the word;3 It seemed good to me also, having had perfect understanding of all things from the very first, to write unto thee in order, most excellent Theophilus,4 That thou mightest know the certainty of those things, wherein thou hast been instructed.
2 Pet. 1:16 For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were EYEWITNESSES of his majesty.17 For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased.18 And this voice which came from heaven we heard, when WE WERE WITH HIM in the holy mount.
1 John 1:1 That which was from the beginning, WHICH WE HAVE HEARD, WHICH WE HAVE SEEN WITH OUR OWN EYES, which we have LOOKED UPON, and OUR HANDS have handled, of the Word of life;2 (For the life was manifested, and we have SEEN IT, and bear WITNESS, and shew unto you that eternal life, which was with the Father, and was manifested unto us;)3 That WHICH WE HAVE SEEN AND HEARD declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ.4 And these things write we unto you, that your joy may be full.5 This then is the message which we have heard of him, and declare unto you, that God is light, and in him is no darkness at all.
John was a witness to the situation with Thomas, and John also ends his gospel by declaring that he indeed is one of the disciples and can therefore testify about the things which he had just described:
John 20:24 But Thomas, one of the twelve, called Didymus, was not with them when Jesus came.25 The other disciples therefore said unto him, We have seen the Lord. But he said unto them, Except I shall see in his hands the print of the nails, and put my finger into the print of the nails, and thrust my hand into his side, I will not believe.26 And after eight days again his disciples were within, and Thomas with them: then came Jesus, the doors being shut, and stood in the midst, and said, Peace be unto you.27 Then saith he to Thomas, Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing.28 And Thomas answered and said unto him, My Lord and my God.29 Jesus saith unto him, Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed.30 And many other signs truly did Jesus in the presence of his disciples, which are not written in this book:31 But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.
John 21:1 After these things Jesus shewed himself again to the disciples at the sea of Tiberias; and on this wise shewed he himself.2 There were together Simon Peter, and Thomas called Didymus, and Nathanael of Cana in Galilee, and the sons of Zebedee, and two other of his disciples.—14 This is now the third time that Jesus shewed himself to his disciples, after that he was risen from the dead.—24 THIS IS THE DISCIPLE WHICH TESTIFIETH OF THESE THINGS, and wrote these things: and we know that his testimony is true.25 And there are also many other things which Jesus did, the which, if they should be written every one, I suppose that even the world itself could not contain the books that should be written. Amen.
The book of Acts (written by Luke) is of course replete with examples of how eyewitnesses perceived Jesus. Examples:
Acts 1:1 The former treatise have I made, O Theophilus, of all that Jesus began both to do and teach,2 Until the day in which he was taken up, after that he through the Holy Ghost had given commandments unto the apostles whom he had chosen:3 To whom also he shewed himself alive after his passion by many infallible proofs, being seen of them forty days, and speaking of the things pertaining to the kingdom of God:4 And, being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me.
Acts 10:7 That word, I say, ye know, which was published throughout all Judaea, and began from Galilee, after the baptism which John preached;38 How God anointed Jesus of Nazareth with the Holy Ghost and with power: who went about doing good, and healing all that were oppressed of the devil; for God was with him.39 And we are WITNESSES OF ALL THINGS WHICH HE DID both in the land of the Jews, and in Jerusalem; whom they slew and hanged on a tree:40 Him God raised up the third day, and shewed him openly;41 Not to all the people, but unto witnesses chosen before God, even to us, who did eat and drink with him after he rose from the dead.42 And he commanded us to preach unto the people, and to testify that it is he which was ordained of God to be the Judge of quick and dead.43 To him give all the prophets witness, that through his name whosoever believeth in him shall receive remission of sins.
If you know Swedish you can read about early non-biblical writers confirming Jesus, in this article.