1 John 4:20-21

²⁰ If someone says, “I love God,” and hates his brother, he is a liar; for he who does not love his brother whom he has seen, how can he love God whom he has not seen?

²¹ And this commandment we have from Him: that he who loves God must love his brother also.

If someone says (v.20) — See 1 John 1:6, 8, 10; 2:4, 6, 9, etc. It is plainly characteristic of this Epistle, and is indicative of the many evil conditions and influences which were at work, to the detriment of the spiritual life of believers. Against this mere professionalism they were to be on their guard, and particularly the profession here mentioned, of showing love to God while denying it to our brother. If we refrain from showing love to our brother, however much we may know of the doctrine of love to God, there is no real comprehension or appreciation of it, because there is no response on our part to His love (v.19). … Such a person is a liar, and his character bears no resemblance to that of God. He neither knows the nature of God, nor does he enjoy communion with Him (v.16). He does not know God (v.8) nor has he been born of Him (v.7). His spirit is not the Spirit of truth but the spirit of error (v.6). — Vine, pages 88-89.

Again, it is very important to remember that John was writing to those saved under the kingdom gospel who had the law of God written on their hearts. We should also love others as a response to God’s love for us, but in the age of grace, this love is not a requirement for, or evidence of, salvation.

has seen (v.20) — The perfect tense expresses an abiding condition. The brother is viewed not as one simply whom we can see but as one who is constantly before our eyes. — Vine, page 89.

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how can he love (v.20) — It is not simply that what is invisible is more difficult to love than what is visible, but that the absence of practical love to one’s brother in the material circumstances of every-day life rules out the possibility of love to God; that is essentially a matter of faith, as being exercised towards Him who is invisible. There is a connection between this verse and what is said of the invisibility of God in v.12. — Vine, page 89.

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[In v.21] what has been regarded as spiritually inevitable in the preceding verse, is put as a distinct command. Not only is love to God impossible where we do not love our brother, but love to our brother is binding upon us. — Vine, page 89.