“He speaks of this”: that is, of Christ’s Second Coming, the end of the world, the events connected with them, and the instructions and exhortations concerning firmness in faith, constancy in good works, and so forth.
“In which there are some things hard to understand”: the phrase “in which” may refer either to the epistles, in which there are things hard to understand, or to the subjects of the teaching.
It refers more to the latter than to the former, especially to all subjects of doctrine concerning the last times and events.
“Some things hard to understand”: not because of the essence of the teaching itself, which, as truth itself, is simple and generally accessible to all. It is because of the special depth of the mysteries of faith.
As mysteries, they do not yield to the comprehension of the mind. Such are, for example, the mystery of the end of the world, Christ’s Second Coming, of which the apostle speaks, and the like.
Instead of submitting to faith and accepting these things in simplicity of mind and heart, the ignorant and unstable twist this difficult material. They distort it by reinterpretation and thereby change its true meaning, corrupting the teaching of truth.
It seems that the apostle has more in view those who have been led into error and are being led astray than those who lead astray, the false teachers and mockers.
To characterize the latter, the apostle would seem to have used stronger expressions, as in 2 Pet. 2:14 and similar passages.
Yet he may also have had in view the false teachers themselves, judging by the strong expression “to their own destruction.” The apostle uses this several times in this same epistle to characterize the false teachers themselves (2 Pet. 2:1, 3).
They act in this way, that is, they twist or distort the apostolic epistles and passages from them to their own destruction.
An incorrect or distorted interpretation, being contrary to the sound teaching of faith, can and must be accompanied for them by deprivation of participation in Christ’s Kingdom, or by separation from the Church (Gal 1:8), and by deprivation of eternal blessedness.
“As they do the other Scriptures”: this probably means partly the apostolic writings, both the Gospel writings and those properly called apostolic, and partly the Old Testament writings. Above all it means the prophetic writings, to which the apostle also points in this epistle (2 Pet. 3:2; 1:20).