Webster's Dictionary defines a "witness" as one who saw, or can give a firsthand account of something; to bear firsthand testimony; or give evidence. The apostles who were with Jesus Christ after his death and resurrection could give a firsthand account of that which they saw. However, in this verse God says that after holy spirit, power from on high, was come they would be witnesses.
When Christ ascended into heaven and sat down at the right hand of God our salvation was completely accomplished. God gave him the gift of holy spirit to distribute to mankind on the day of Pentecost. It is that holy spirit in operation in a believer's life that is the evidence or witness that Jesus Christ did come, did die for us, was raised from the dead, and is seated at God's right hand.
And when the day of Pentecost was fully come, they were all with one accord in one place.
And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting.
And there appeared unto them cloven tongues like as of fire, and it sat upon each of them.
And they were all filled with the Holy Ghost (holy spirit), and began to speak with other tongues, as the Spirit gave them utterance.
The apostles received the gift of holy spirit on the day of Pentecost and they spoke in tongues. They were seated in the Temple and a multitude came together to hear this wonderful witness. The completeness of Christ's accomplishments was witnessed to as these believers spoke in tongues. Later Peter stood up and explained:
This Jesus hath God raised up, whereof we all are witnesses.
Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Ghost (holy spirit), he hath shed forth this, which ye now see and hear.
The apostles were firsthand witnesses of Christ's death and resurrection. When they spoke in tongues on the day of Pentecost they were witnesses of the fact that Christ was exalted and seated at the right hand of God and that he had "shed forth this, which ye now see and hear."
And with great power gave the apostles witness of the resurrection of the Lord Jesus: and great grace was upon them all.
The God of our fathers raised up Jesus, whom ye slew and hanged on a tree.
Him hath God exalted with his right hand to be a Prince and a Saviour, for to give repentance to Israel, and forgiveness of sins.
And we are his witnesses of these things; and so is also the Holy Ghost (holy spirit), whom God hath given to them that obey him.
It was the great power of God in operation in the lives of the apostles after they received holy spirit that gave witness to the death, resurrection, and exaltation of Jesus Christ.
For as many as are led by the Spirit of God, they are the sons of God.
For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father.
The Spirit itself beareth witness with our spirit, that we are the children of God.
When we confess with our mouth the Lord Jesus and believe in our heart that God raised him from the dead (Romans 10:9-10) we also are saved, born again, given holy spirit. As we speak in tongues and operate that spirit within, our spirit beareth witness with The Spirit that we are the children of God. It is the holy spirit in manifestation in our lives that is the witness or evidence that we are His children.
1 John 5:10a
He that believeth on the Son of God hath the witness in himself . . .
Since you believed you have the evidence
or witness in yourself that you are born again of His spirit. Walk
in the greatness of the power of God and speak much in tongues.