5 Once more there is the familiar difficulty of determining whether the phrase refers (1) to what precedes or (2) to what follows. Here, because ἐν τούτῳ (en toutō) is followed by a clause introduced by ὅταν (hotan) which appears to be related, it is best to understand ἐν τούτῳ as referring to what follows. The following ὅταν clause is epexegetical to ἐν τούτῳ, explaining how we know that we love God’s children: “by this we know that we love God’s children, whenever we love God and keep his commandments.”

In other versions

By this we know that we love the children of God, when we love God and obey his commandments.

This is how we know that we love the children of God: by loving God and carrying out his commands.

This is how we know that we love God’s children: when we love God and obey his commands.

We know we love God’s children if we love God and obey his commandments.

By this we know that we love the children of God: whenever we love God and keep his commandments.

By this we know that we love the children of God, when we love God, and keep his commandments.

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