1 John 4:20

If anyone says, “I love God,” and hates (works against) his [Christian] brother he is a liar; for the one who does not love his brother whom he has seen, cannot love God whom he has not seen.

1 John 2:4

Whoever says, “I have come to know Him,” but does not habitually keep [focused on His precepts and obey] His commandments (teachings), is a liar, and the truth [of the divine word] is not in him.

1 John 2:9

The one who says he is in the Light [in consistent fellowship with Christ] and yet habitually hates (works against) his brother [in Christ] is in the darkness until now.

1 John 3:17

But whoever has the world’s goods (adequate resources), and sees his brother in need, but has no compassion for him, how does the love of God live in him?

1 John 4:12

No one has seen God at any time. But if we love one another [with unselfish concern], God abides in us, and His love [the love that is His essence abides in us and] is completed and perfected in us.

1 John 2:11

But the one who habitually hates (works against) his brother [in Christ] is in [spiritual] darkness and is walking in the darkness, and does not know where he is going because the darkness has blinded his eyes.

1 Peter 1:8

Though you have not seen Him, you love Him; and though you do not even see Him now, you believe and trust in Him and you greatly rejoice and delight with inexpressible and glorious joy,

1 John 1:6

If we say that we have fellowship with Him and yet walk in the darkness [of sin], we lie and do not practice the truth;

Treasury of Scripture Knowledge

Summary

If a man say, I love God, and hateth his brother, he is a liar: for he that loveth not his brother whom he hath seen, how can he love God whom he hath not seen?

Bible References

A man

1 John 2:4
Whoever says, “I have come to know Him,” but does not habitually keep [focused on His precepts and obey] His commandments (teachings), is a liar, and the truth [of the divine word] is not in him.
1 John 3:17
But whoever has the world’s goods (adequate resources), and sees his brother in need, but has no compassion for him, how does the love of God live in him?

Not

1 John 4:12
No one has seen God at any time. But if we love one another [with unselfish concern], God abides in us, and His love [the love that is His essence abides in us and] is completed and perfected in us.

General references

Matthew 22:40
The whole Law and the [writings of the] Prophets depend on these two commandments.”
Matthew 25:42
for I was hungry, and you gave Me nothing to eat; I was thirsty, and you gave Me nothing to drink;
John 13:35
By this everyone will know that you are My disciples, if you have love and unselfish concern for one another.”
John 14:15
“If you [really] love Me, you will keep and obey My commandments.
Hebrews 13:1
Let love of your fellow believers continue.