This verse draws the conclusion from verse 5: since Christ and sin have nothing in common, fellowship with Christ cannot go together with sin. The verse may be introduced by some word or phrase meaning “consequently.”
No one who abides in him sins, or ‘every one who abides in him (or in Christ) does not sin,’ ‘if a person abides in him/Christ, he does not sin.’ The present tense of “to abide” in this and of “to sin” in the next clause characterizes the sentence as stating a general truth. For “to abide in” and “to sin,” see comments on 2.6 and 1.10, respectively.
No one who sins has either seen him or known him states the opposite of verse 6a: a person who sins proves by that very fact that Christ is a stranger to him, whom he has never seen or known.
Because of the reverse parallel structure of this and the preceding clause, one might expect here “abides in him,” echoing the first part of 6a. The reason why John preferred to employ the verbs “to see” and “to know” instead was probably to refute the false teachers, who used to boast of having the true vision and knowledge of Christ. This boast must be false, John points out, since they are persons who sin (according to 1.8). The perfect tense of has … seen … or known is to show that the present situation is viewed as the result of previous events.
Seen him is used here metaphorically. It refers to seeing with the eye of faith, realizing that Christ is the revelation of God (John 14.7, 9). Sometimes the corresponding verb does not allow this figurative use. Then one has to use another verb of perception; for example, ‘to understand,’ ‘to find,’ or ‘to know-by-face’ (in one American Indian language).
Known him: for the verb see comments on 2.3, where, however, the object is not Christ but God. The American Indian language just quoted uses here a verb meaning ‘to know-by-mind.’
Quoted with permission from Haas, C., de Jonge, M. and Swellengrebel, J.L. A Handbook on The First Letter of John. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .