Charles Ellicott Commentary John 15:27

"and ye also bear witness, because ye have been with me from the beginning." — John 15:27 (ASV)

And you also shall bear witness.—The tense is present, and you also bear witness; or, and you also are witnesses. (Compare Notes on Luke 24:48-49.) The Apostles themselves distinguished between their own witness of things which had come within their own experience and the witness borne by the power of the Holy Spirit, of which the Day of Pentecost was the first great instance. (Compare Acts 5:32.)

Because you have been with me from the beginning.—Compare John 1:7; and Notes on Acts 1:21-22. The “beginning,” of course, means the beginning of the Messianic teaching and works of which they were to be witnesses.