Jamieson, Fausset and Brown on Jude 1:17

17. But; beloved ... ye--in contrast to those reprobates, Jude 20, again.

remember--implying that his readers had been contemporaries of the apostles. For Peter uses the very same formula in reminding the contemporaries of himself and the other apostles.

spoken before--spoken already before now.

the apostles--Peter (see on 2Pe 3:2, 3), and Paul before Peter (Ac 20:29; 1Ti 4:1; 2Ti 3:1). Jude does not exclude himself from the number of the apostles here, for in Jude 18, immediately after, he says, "they told You," not us (rather as Greek, "used to tell you" implying that Jude's readers were contemporaries of the apostles, who used to tell them).