we know.

John 19:35
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.

1 John 1:1, 2
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 5:6
This is the One who came by water and blood, Jesus Christ--not by water alone, but by water and blood. And it is the Spirit who testifies to this, because the Spirit is the truth.

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.