All who keep his commandments: here his undoubtedly refers to God.
Abide in him, or ‘are and remain in God,’ compare comments on 2.6, group (a).
And he in them, that is, ‘and God abides in them’; for this expression see the next entry. The same elliptical construction is found in 4.13.
By this we know that he abides in us, by the Spirit which he has given us: the introductory words by this point forward to the phrase by the Spirit which he has given us. They are used in order to place full emphasis upon that phrase; compare comments on “by this we may be sure” in 2.3. The clause/ structure may have to be changed; for example, ‘How do we know that he abides in us? (We know it) by the Spirit he has given us,’ ‘He has given us the Spirit. Thereby we know that he abides in us,’ ‘The Spirit, his gift to us, proves to us (or causes us to know) that he abides in us.’
For he abides in us compare comments on 2.6, group (b). The clause is a strong assertion of God’s continuous fellowship and presence with the believer. The verb may be rendered by ‘to be continually in/with,’ ‘to stay (and work) in/with.’ The assertion is stated here for the first time in the Letter, and again in 4.12 and 15-16. It must probably be viewed against the background of 3.1, where the believers are called children of God.
By the Spirit, literally “out of the Spirit,” marks the Spirit as the source of the knowledge. And the source of knowledge is at the same time its cause; hence causative renderings of the clause are sometimes given; see one of the examples given above.
The Spirit which he has given us: this closing clause of Part Two forms a transition to 4.1-6, which speaks of the Spirit. Several versions prefer to change the clause structure into ‘the fact that he has given us the Spirit,’ making it almost identical to what is said in 4.13. This is a defensible adjustment of the phrase. The noun is often better specified, for example, by ‘his Spirit’ or ‘the Holy Spirit.’ For its rendering see comments on 4.1f.
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 .