Jamieson, Fausset and Brown on 2 Peter 3:11

11. Your duty, seeing that this is so, is to be ever eagerly expecting the day of God.

then--Some oldest manuscripts substitute "thus" for "then": a happy refutation of the "thus" of the scoffers, 2Pe 3:4 (English Version, "As they were," Greek, "thus").

shall be--Greek, "are being (in God's appointment, soon to be fulfilled) dissolved"; the present tense implying the certainty as though it were actually present.

what manner of men--exclamatory. How watchful, prayerful, zealous!

to be--not the mere Greek substantive verb of existence (einai), but (huparchein) denoting a state or condition in which one is supposed to be [Tittmann]. What holy men ye ought to be found to be, when the event comes! This is "the holy commandment" mentioned in 2Pe 3:2.

conversation ... godliness--Greek, plural: behaviors (towards men), godlinesses (or pieties towards God) in their manifold modes of manifestation.