John 15:26 KJV — But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me:
John 15:27 KJV — And ye also shall bear witness, because ye have been with me from the beginning.
Who shal testify or bear witness?
Per Scripture, the Spirit of truth as well as the disciples. No problem with the unbelievers as they are in darkness so that it is the task of the disiples to Christ , bear witness, testify that Christ is the Light.
What you are trying to insert is John 14, but thats another story
A biblical definition of the word ‘testify’ s in the immediate context following the phrase “bearing witness” v.27, thus we have the witness. The witness is to tell the truth about what he has seen firsthand. The role of the Holy Spirit is to attest to the truth and his disciples at the same time will be empowered once the Spirit of truth comes upon them as in Acts 1:8. The Spirit who is called Comforter will not only “ come alongside” and solace in the times of adversity, trials or persecutions as they were hated but also to ‘come along’ with them empowering the disciples to testify the truth of the gospel. So when the empowered disciples begin to witness, the Holy Spirit comes alongside them testifying the truth to the hearers while the gospel truth or the scripture of truth is being preached.
Technically, the Holy Spirit will not testify to believers but will empower them. You argue that the Holy Spirit of promise will testify to the disciples so that the disciple is to testify, which is not. Jesus said they would testify because they already had been with Jesus at the beginning of his earthly ministry. John boldly spoke in this manner and Luke did as well. Here’s what they said:
1 John 1:1-2
King James Version
1 That which was from the beginning, which we have heard, which we have seen with our 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
Acts 1:3
King James Version
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: