A.T. Robertson Commentary John 15:27

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

And ye also bear witness (κα υμεις δε μαρτυρειτε). Present active indicative or imperative (do ye bear witness), same form of μαρτυρεω. "Ye also" as well as the Holy Spirit, ye also when filled with and taught by the Holy Spirit the things concerning Jesus. It is here that Christians fail most.

Have been (εστε). Progressive present of ειμ, "are with me from the beginning of my ministry as in 14:9. They were chosen to be with Christ .