True Christians are people characterised by love. This is a running theme in the New Testament, and especially in the writings of the Apostle John. Just consider a sampling of passages from John's first letter.
2:9-11: Whoever says he is in the light and hates his brother is still in darkness. Whoever loves his brother abides in the light, and in him there is no cause for stumbling. But whoever hates his brother is in the darkness and walks in the darkness, and does not know where he is going, because the darkness has blinded his eyes.
A person who lacks love walks in darkness.
3:10: By this it is evident who are the children of God, and who are the children of the devil: whoever does not practice righteousness is not of God, nor is the one who does not love his brother.
A person who lacks love is a child of the devil, not of God.
3:14-15: We know that we have passed out of death into life, because we love the brothers. Whoever does not love abides in death. Everyone who hates his brother is a murderer, and you know that no murderer has eternal life abiding in him.
A person who lacks love abides in death, is a murderer, and has no eternal life abinding within.
3:16-18: By this we know love, that he laid down his life for us, and we ought to lay down our lives for the brothers. But if anyone has the world's goods and sees his brother in need, yet closes his heart against him, how does God's love abide in him? Little children, let us not love in word or talk but in deed and in truth.
A person who lacks love does not have the love of God within.
4:7-8: Beloved, let us love one another, for love is from God, and whoever loves has been born of God and knows God. Anyone who does not love does not know God, because God is love.
A person who lacks love does not know God and is not born of God.
4:20-21: If anyone says, "I love God," and hates his brother, he is a liar; for he who does not love his brother whom he has seen cannot love God whom he has not seen. And this commandment we have from him: whoever loves God must also love his brother.
A person who lacks love for others (while claiming to love God) is a liar.
Now look at the cumulative force of all that John says. When you put all of those passages together, they say that if someone lacks love, he/she:
1) is in darkness;
2) is a child of the devil and not of God,
3) abides in death;
4) is a murderer,
5) has no eternal life abiding within,
6) doesn't have the love of God within,
7) does not know God,
8) is not born of God,
9) [if he or she claims to love God] is a liar.
Looks like love is important, huh?