To put you in remembrance (hupomnēsai). See 2Pe 1:12 hupomimnēskein (present active infinitive there, first aorist active infinitive here).
Though ye know all things once for all (eidotas hapax panta). Concessive perfect (sense of present) active participle as in 2Pe 1:12, but without kaiper.
The Lord (kurios). Some MSS. add Iēsous. The use of kurios here is usually understood to mean the Lord Jesus Christ, as Clement of Alex. (Adumbr. p. 133) explains, Exo 23:20, by ho mustikos ekeinos aggelos Iēsous (that mystical angel Jesus). For the mystic reference to Christ see 1Co 10:4, 1Co 10:9; Heb 11:26. Some MSS. here add theos instead of Iēsous.
Afterward (to deuteron). Adverbial accusative, “the second time.” After having saved the people out of Egypt.
Destroyed (apōlesen). First aorist active indicative of apollumi, old verb, to destroy.
Them that believed not (tous mē pisteusantas). First aorist active articular participle of pisteuō. The reference is to Num 14:27-37, when all the people rescued from Egypt perished except Caleb and Joshua. This first example by Jude is not in 2 Peter, but is discussed in 1Co 10:5-11; Heb 3:18-4:2.