Q: How doth Christ execute the office of a prophet?
A: Christ executeth the office of a prophet, in revealing to us, by his word and Spirit, the will of God for our salvation.

John 1:1, 4. In the beginning was the Word, and the Word was with God, and the Word was God. . . In him was life; and the life was the light of men.
John 15:15. Henceforth I call you not servants; for the servant knoweth not what his lord doeth: but I have called you friends; for all things that I have heard of my Father I have made known unto you.
John 20:31. But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.
2 Peter 1:21. For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.
John 14:26. But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.

Westminster Project - Francis Beattie on the Prophetic Role of the Mediator