To put you in remembrance (υπομνησα). See 2 Peter 1:12 υπομιμνησκειν (present active infinitive there, first aorist active infinitive here).

Though ye know all things once for all (ειδοτας απαξ παντα). Concessive perfect (sense of present) active participle as in 2 Peter 1:12, but without καιπερ.

The Lord (κυριος). Some MSS. add Ιησους. The use of κυριος here is usually understood to mean the Lord Jesus Christ, as Clement of Alex (Adumbr. p. 133). explains, Exodus 23:20, by ο μυστικος εκεινος αγγελος Ιησους (that mystical angel Jesus). For the mystic reference to Christ see 1 Corinthians 10:4; 1 Corinthians 10:9; Hebrews 11:26. Some MSS. here add θεος instead of Ιησους.

Afterward (το δευτερον). Adverbial accusative, "the second time." After having saved the people out of Egypt.

Destroyed (απωλεσεν). First aorist active indicative of απολλυμ, old verb, to destroy.

Them that believed not (τους μη πιστευσαντας). First aorist active articular participle of πιστευω. The reference is to Numbers 14:27-37, when all the people rescued from Egypt perished except Caleb and Joshua. This first example by Jude is not in II Peter, but is discussed in 1 Corinthians 10:5-11; Hebrews 3:18-4.