Translation:

17. Can they be (like) those who accept a Clear (Sign) from their Lord and whom a witness from Himself doth teach as did the Book of Moses before it a guide and a mercy? They believe therein; but those of the Sects that reject it the Fire will be their promised meeting place. Be not then in doubt thereon: for it is the Truth from thy Lord: yet many among men do not believe!

Notes (Tafseer)

1512. "A witness from Himself': i.e., the Book which was given to Al-Mustafa, the Holy Qur-an, which is compared to the original Revelation given to Moses. We make no difference between one true and genuine Message and another, nor between one apostle and another,-for they all come from the One True God.

1513. "Guide": the Arabic word here is Imam, a leader, a guide, one that directs to the true Path. Such a direction is an instance of the Mercy and Goodness of Allah to man. The Qur-an and the Prophet Muhammad are also called, each, a Guide and a Mercy, and so are these epithets applicable to previous Books and Prophets.