In the following "testify, testifying, testimony" are what AV translates with "witness," etc.
John 8:12-18 (CLV)...
12 Again, then, Jesus speaks to them, saying, "I am the Light of the world. He who is following Me should under no circumstances be walking in darkness, but will be having the light of life."
13 The Pharisees, then, said to Him, "You are testifying concerning yourself. Your testimony is not true!"
14 Jesus answered and said to them, "And if I should be testifying concerning Myself, true is My testimony, for I am aware whence I came and whither I am going, yet you are not aware whence I am coming or whither I am going.
15 You are judging according to the flesh; I am not judging anyone.
16 And yet if ever I should be judging, My judging is true, for not alone am I, but I and the Father Who sends Me.
17 Yet in this law, also, of yours it is written that the testimony of two men is true.
18 I am the One testifying concerning Myself, and the Father Who sends Me is testifying concerning Me."
John 5:30-44 (CLV)...
30 "I can not do anything of Myself. According as I am hearing am I judging; and My judging is just, for I am not seeking My will, but the will of Him Who sends Me.
31 "If I should be testifying concerning Myself, is My testimony not true?
32 There is another who is testifying concerning Me, and I am aware that the testimony which he is testifying concerning Me is true.
33 You have dispatched to John, and he has testified to the truth.
34 Yet I am not getting the testimony from man, but I am saying these things that you may be saved.
35 He was a lamp, burning and appearing, yet you want to exult an hour in its light.
36 "Now I have a testimony greater than John's. For the works which the Father has given Me that I should be perfecting them, the works themselves which I am doing are testifying concerning Me that the Father has commissioned Me.
37 And the Father Who sends Me, He has testified concerning Me. Neither have you ever heard His voice nor a perception of Him have you seen.
38 And His word you do not have remaining in you, for that One Whom He commissions, this One you are not believing.
39 "Search the scriptures, for in them you are supposing you have life eonian, and those are they which are testifying concerning Me,
40 and not willing are you to come to Me that you may have life.
41 "Glory from men I am not getting.
42 But I know you, that you have not the love of God in yourselves.
43 I have come in the name of My Father, and you are not getting Me. If another should be coming in his own name, him you will get.
44 How can you believe, getting glory from one another, and are not seeking the glory which is from God alone?
17 Yet in this law, also, of yours it is written that the testimony of two men is true.
18 I am the One testifying concerning Myself, and the Father Who sends Me is testifying concerning Me."
45 "Be not supposing that I shall be accusing you to the Father. He who is accusing you to the Father is Moses, on whom you rely.
46 For if you believed Moses, you would believe Me, for he writes concerning Me.
47 Now if you are not believing his writings, how shall you be believing My declarations?"
The first two or three witnesses is Jesus and His Father, along with Scripture. As well, the Holy Spirit bears witness of Jesus. For each of us, my own self with God is the two witnesses that establish His presence presence in the earh. This needs to be rigorously asserted. For the world, the more that are in agreement the more certain that thing is. With God it only takes Himself. For each of us, it only takes God and me.
Matthew 18:15-20 (CLV)...
15 "Now, if ever your brother should be sinning, go and expose him between you and him alone. If ever he should be hearing you, you gain your brother.
16 Yet if ever he should not be hearing, take still one or two others along with you, that at the mouth of two witnesses, or of three, every declaration may be made to stand.
17 Now, if ever he should be disobeying them, tell it to the ecclesia. Now, if ever he should be disobeying the ecclesia also, let him be to you even as the man of the nations, and the tribute collector.
18 "Verily, I am saying to you, Whatsoever you should be binding on the earth shall be those things having been bound in the heavens, and whatsoever you should be loosing on the earth shall be those loose in heaven.
19 "Again, verily, I am saying to you that, if ever two of you should be agreeing on the earth concerning any matter, whatsoever it is they should be requesting shall be coming to them from My Father Who is in the heavens.
20 For where two or three are, gathered in My name, there am I in the midst of them."
This is Jesus' complete doctrine of the Church (ecclesiology.)