The one who saw it has testified to this, and his testimony is true. He knows that he is telling the truth, so that you also may believe.

John 15:27
And you also must testify, because you have been with Me from the beginning.

John 20:31
But these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in His name.

John 21:24
This is the disciple who testifies to these things and who has written them down. And we know that his testimony is true.

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 gazed upon and touched with our own hands--this is the Word of life.

1 John 1:2
And this is the life that was revealed; we have seen it and testified to it, and we proclaim to you the eternal life that was with the Father and was revealed to us.

1 John 1:3
We proclaim to you what we have seen and heard, so that you also may have fellowship with us. And this fellowship of ours is with the Father and with His Son, Jesus Christ.

3 John 1:12
Demetrius has received a good testimony from everyone, and from the truth itself. We also testify for him, and you know that our testimony is true.

And he that saw it bore record, and his record is true: and he knows that he said true, that you might believe.