That ye may be mindful. By these words he intimates that we have enough in the writings of the prophets, and in the gospel, to stir us up, provided we be as diligent as it behoves us, in meditating on them; and that our minds sometimes contract a rust, or become bedimmed through darkness, is owing to our sloth. That God may then continually shine upon us, we must devote ourselves to that study: let our faith at the same time acquiesce in witnesses so certain and credible. For when we have the prophets and apostles agreeing with us, nay, as the ministers of our faith, and God as the author, and angels as approvers, there is no reason that the ungodly, all united, should move us from our position. By the commandment of the apostles he means the whole doctrine in which they had instructed the faithful. [1]

Footnotes

1 - The construction of the passage is as follows: -- "In both which I, by admonition, arouse your sincere mind to remember the words, aforetime spoken by the holy prophets, and the doctrine of us, the apostles of our Lord and Savior." The verb mnesthonai is connected with "arouse;" and it is in this tense used actively as well as passively. See Matthew 26:75, and Acts 10:31. There is in the noun entole, a metonymy, the commandment for what was commanded to be taught, the doctrine. It has this meaning, according to Schleusner, in John 12:50, and in this Epistle, chapter 2:21. -- Ed

That ye may be mindful of the words - Of the doctrines, the truths; the prophetic statements. Jude Jde 1:18 says that it had been foretold by the apostles, that in the last days there would be scoffers. Peter refers to the instructions of the apostles and prophets in general, though evidently designing that his remarks should bear particularly on the fact that there would be scoffers.
Which were spoken before by the holy prophets - The predictions of the prophets before the advent of the Saviour, respecting his character and work. Peter had before appealed to them 2-Peter 1:19-21, as furnishing important evidence in regard to the truth of the Christian religion, and valuable instruction in reference to its nature. See the notes at that passage. Many of the most important doctrines respecting the kingdom of the Messiah are stated as clearly in the Old Testament as in the New Testament (compare Isaiah 53:1-12), and the prophecies therefore deserve to be studied as an important part of divine revelation. It should be added here, however, that when Peter wrote there was this special reason why he referred to the prophets, that the canon of the New Testament was not then completed, and he could not make his appeal to that. To some parts of the writings of Paul he could and did appeal 2-Peter 3:15-16, but probably a very small part of what is now the New Testament was known to those to whom this epistle was addressed.
And of the commandment of us the apostles of the Lord and Saviour - As being equally entitled with the prophets to state and enforce the doctrines and duties of religion. It may be observed, that no man would have used this language who did not regard himself and his fellow apostles as inspired, and as on a level with the prophets.

That ye may be mindful,.... This is an explanation of the above mentioned end of his writing this and the other epistle; which was, that those saints might be mindful of two things more especially:
of the words which were spoken before by the holy prophets; that is, the prophets of the Old Testament, who were holy men of God, and therefore their words are to be regarded, and retained in memory; the Gospel itself was spoken by them, and so was Christ, and the things relating to his person and offices, and to his incarnation, sufferings, and death, and the glory that should follow; and indeed the apostles said no other than what they did, only more clearly and expressly; and particularly many things, were said by them concerning the second coming of Christ to judge the world, and destroy it, and to prepare new heavens and a new earth for his people, which is what the apostle has chiefly in view; see Jde 1:14;
and of the commandment of us the apostles of the Lord and Saviour; that is, Jesus Christ, as Jde 1:17 expresses it, and the Ethiopic version adds here; and which likewise, and also the Syriac version, and some ancient copies, read, "our Lord and Saviour", and omit the us before the apostles; by whom are meant the twelve apostles of Christ, of which Peter was one, and therefore says, "us the apostles"; though the Vulgate Latin and Arabic versions, and the Complutensian edition, read "your apostles", and so the Alexandrian copy; but the former is the received reading: now "the commandment" of these intends either the Gospel in general, so called because it was the commandment of our Lord to his apostles to preach it; and therefore the word "commandment", in the original, stands between "us the apostles", and "the Lord and Saviour", as being the commandment of the one to the other; unless it can be thought any regard is had to the new commandment of love, or that of faith, inculcated both by Christ and his apostles; John 13:34; or rather, particularly the instructions, directions, and predictions of the apostles concerning the second coming of Christ, and what should go before it, as appears from the following words, and the parallel place in Jde 1:17, the words of the prophets and apostles being here put together, show the agreement there is between them, and what regard is to be had to each of them, and to anything and every thing in which they agree.

prophets--of the Old Testament.
of us--The oldest manuscripts and Vulgate read, "And of the commandment of the Lord and Saviour (declared) by YOUR apostles" (so "apostle of the Gentiles," Romans 11:13) --the apostles who live among you in the present time, in contrast to the Old Testament "prophets."

Be the more mindful thereof, because ye know scoffers will come first - Before the Lord comes. Walking after their own evil desires - Here is the origin of the error, the root of libertinism. Do we not see this eminently fulfilled?