Though ye once knew this] RV 'though ye know all things once for all,' i.e. because they are Christians, and have learnt the whole Christian creed (cp. Hebrews 6:2); or perhaps (a deeper thought) because they have as Christians 'the Spirit in their mind'(cp. Ephesians 4:17; Ephesians 4:23), and 'need not that any man teach them' (1 John 2:27).
A curious reading of some MSS is noticed in RM—'Jesus' for the Lord. It recalls Hebrews 4:8, as the rest of the v. does Hebrews 3:17-19.