And ye also shall bear witness, because ye have been with me from the beginning.

People's New Testament

15:27 Ye also shall bear witness. The apostles had been with him from the beginning, and knew all the facts. If he was true, they knew it; if he stilled the waves and raised the dead, they knew it; if he rose from the tomb, they knew it (see Ac 1:21,22). They bore witness to him by word and left their words to us. They bore witness by life, gave up all for him and died for their testimony. Such testimony is the strongest human testimony ever offered to any fact.

Jamieson-Fausset-Brown Bible Commentary

27. ye also shall bear witness-rather, "are witnesses"; with reference indeed to their future witness-bearing, but putting the emphasis upon their present ample opportunities for acquiring their qualifications for that great office, inasmuch as they had been "with Him from the beginning." (See on [1865]Lu 1:2).

Matthew Henry's Concise Commentary

15:26,27. The blessed Spirit will maintain the cause of Christ in the world, notwithstanding the opposition it meets with. Believers taught and encouraged by his influences, would bear testimony to Christ and his salvation.