1 John 4:20

If a person says, "I love God," yet [in reality] hates his brother, he is a liar. For the person who does not love his brother, whom he has seen, cannot [possibly] love God, whom he has never seen.

1 John 2:4

The person who says, "I have come to know Him," but does not obey what He commands, is lying [by such a profession], and the truth [about the matter] is not in his heart.

1 John 2:9

The person who claims to be living in [harmony with] the light [i.e., by obeying God's truth], and [yet] hates his brother is still [living] in the darkness [of sin].

1 John 3:17

Now whoever has [a sufficient amount of] life's possessions and [then] observes one of his [Christian] brothers who does not have enough to get by on, but does not feel sorry for him [i.e., enough to help him out], surely a love for God does not continue to live in that person's heart.

1 John 4:12

No person has gazed upon God [at any time]; [but] if we love one another, God [continually] lives in our hearts, and His love is made complete within us.

1 John 2:11

But the person who hates his brother is in the darkness [of sin] and he lives in [harmony with] that darkness, and he does not know where he is heading, because the darkness [of sin] has blinded his eyes [to his spiritual condition].

1 Peter 1:8

Although you have not seen Jesus, you [still] love Him and believe in Him, and this causes you to have an indescribable, glorious joy.

1 John 1:6

If we claim to share a relationship with God and yet live in darkness [i.e., in a sinful way], we are lying and are not living a truthful life.

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
The person who says, "I have come to know Him," but does not obey what He commands, is lying [by such a profession], and the truth [about the matter] is not in his heart.
1 John 3:17
Now whoever has [a sufficient amount of] life's possessions and [then] observes one of his [Christian] brothers who does not have enough to get by on, but does not feel sorry for him [i.e., enough to help him out], surely a love for God does not continue to live in that person's heart.

Not

1 John 4:12
No person has gazed upon God [at any time]; [but] if we love one another, God [continually] lives in our hearts, and His love is made complete within us.

General references

Matthew 22:40
The entire law of Moses and the prophets hinge on these two commandments."
Matthew 25:42
For [when] I was hungry, you did not feed me; [when] I was thirsty, you did not give me anything to drink;
John 13:35
If you love one another, then everyone will know by [observing] this that you are my disciples."
John 14:15
If you love me, you will obey my commandments.
Hebrews 13:1
Continue to love [one another] as brothers.