To abide in God is a more common expression with John than to be in God, and marks an advance in thought. The phrase is a favorite one with John. See Joh 15:4 sqq.; Joh 6:56; 1Jo 2:24, 1Jo 2:27, 1Jo 2:28; 1Jo 3:6, 1Jo 3:24; 1Jo 4:12 sq.; 1Jo 4:15 sq. Bengel notes the gradation in the three phrases “to know Him, to be in Him, to abide in Him; knowledge, fellowship, constancy.”
Ought (ὀφείλει)
An obligation, put as a debt. See Luk 17:10, and on debts, Mat 6:12. The word expresses a special, personal obligation, and not as δεῖ must, an obligation in the nature of things. See Joh 20:9, and compare 1Jo 3:16; 1Jo 4:11; 3Jo 1:8.
He (ἐκεῖνος)
Always of Christ in the Epistles of John. See ἐκείνης, referring to ἁμαρτία sin, 1Jo 5:16.