John Calvin Commentary 1 John 5:2

"Hereby we know that we love the children of God, when we love God and do his commandments." — 1 John 5:2 (ASV)

By this we know. In these words, he briefly shows what true love is: that is, love directed towards God. He has until now taught us that there is never true love for God unless our brothers and sisters are also loved, for this is always its effect. But he now teaches us that people are rightly and properly loved when God holds the primacy. And this is a necessary definition, because it often happens that we love people apart from God, as unholy and worldly friendships focus only on personal advantages or other fleeting things. Therefore, just as he first referred to the effect, he now refers to the cause, for his purpose is to show that mutual love should be cultivated in such a way that God is honored.

To the love of God he joins the keeping of the law, and rightly so, for when we love God as our Father and Lord, reverence must necessarily be connected with that love. Besides, God cannot be separated from Himself. Therefore, since He is the fountain of all righteousness and equity, whoever loves Him must necessarily have his heart prepared to render obedience to righteousness. The love of God, then, is not idle or inactive.

But from this passage we also learn what the keeping of the law is. For if we obey God by keeping His commandments only when constrained by fear, we are very far from true obedience. Therefore, the first thing is that our hearts should be devoted to God in willing reverence, and then, that our lives should be conformed to the rule of the law. This is what Moses meant when, in giving a summary of the law, he said:

O Israel, what does the Lord your God require of you, but to love Him and to obey Him? (Deuteronomy 10:12).