John presents these verses as a challenge, a test, to his readers. There are many who say they “love God” or “I have fellowship with God” or “I know God,” and John is saying, "Prove it by loving your brother." The proof that love is real is the action that it produces. It will always help the other person in the long run.
If a person really loves God, he will honor his parents, he will not commit murder, nor will he fornicate, commit adultery, lie, steal, or covet. In other words, he will keep God's Commandments. If a person is really becoming close to God, he will not do any of these things—and not their spiritual counterparts either.
This verse, then, offers a permanently valid test to see whether our religion is the true one or not. Do we love God? Do we love one another? The proof that we love God is that we love our brother with agape love, for that kind of love must have an outlet, or it will not be reciprocated back to God.
The feelings associated with agape love arise as a result of our fellowship with God through experiencing life's events with Him as a dominating influence on our thinking.
John W. Ritenbaugh
Love's Emotional Dimension
Other Forerunner Commentary entries containing 1 John 4:21: