John 21:24

This is the disciple who is giving testimony about these things and has written them down, and we know his testimony is true.

John 19:35

And the person who saw this happen is giving a true testimony about it. He knows it is true [and is giving it] so that you will believe [it, also].

3 John 1:12

Demetrius has been given [a favorable] commendation by everyone, and [is commended] by the truth itself. We [apostles] also give him [a favorable] commendation, and you know that our testimony [about these things] is true.

John 15:27

And you will give testimony about me [also], because you men have been with me from the beginning [of my ministry]."

1 John 1:1-2

We [apostles] proclaim to you [the truth about Jesus], who existed from the beginning [of time]. We have heard [Him] and seen [Him] with our [own] eyes. We looked at Him, and [even] touched Him with our hands. [See John 20:20]. [So, we proclaim that] He is the word of life [i.e., the life-giving Word]!

1 John 5:6

This Jesus is the one whose coming [to earth was introduced] by water [at His immersion. See Matt. 3:13] and [completed] by blood [at His crucifixion, John 19:17-37]. He did not come by the water only, but by the water and by the blood. And it is the Holy Spirit who gives testimony [at both of these events], because the Holy Spirit is [always] the truth. [See John 16:13].

Treasury of Scripture Knowledge

Summary

This is the disciple which testifieth of these things, and wrote these things: and we know that his testimony is true.

Bible References

We know

John 19:35
And the person who saw this happen is giving a true testimony about it. He knows it is true [and is giving it] so that you will believe [it, also].
1 John 1:1
We [apostles] proclaim to you [the truth about Jesus], who existed from the beginning [of time]. We have heard [Him] and seen [Him] with our [own] eyes. We looked at Him, and [even] touched Him with our hands. [See John 20:20]. [So, we proclaim that] He is the word of life [i.e., the life-giving Word]!
1 John 5:6
This Jesus is the one whose coming [to earth was introduced] by water [at His immersion. See Matt. 3:13] and [completed] by blood [at His crucifixion, John 19:17-37]. He did not come by the water only, but by the water and by the blood. And it is the Holy Spirit who gives testimony [at both of these events], because the Holy Spirit is [always] the truth. [See John 16:13].
3 John 1:12
Demetrius has been given [a favorable] commendation by everyone, and [is commended] by the truth itself. We [apostles] also give him [a favorable] commendation, and you know that our testimony [about these things] is true.