54 The ἵνα (hina) clause in 4:21 could be giving (1) the purpose or (2) the result of the commandment mentioned in the first half of the verse, but if it does, the author nowhere specifies what the commandment consists of. It makes better sense to understand this ἵνα clause as (3) epexegetical to the pronoun ταύτην (tautēn) at the beginning of 4:21 and thus explaining what the commandment consists of: “that the one who loves God should love his brother also.”
55 See note on the phrase “fellow Christian” in .

In other versions

And this commandment we have from him: whoever loves God must also love his brother.

And he has given us this command: Anyone who loves God must also love their brother and sister.

And we have this command from him: The one who loves God must also love his brother and sister.

And he has given us this command: Those who love God must also love their fellow believers.

And this is the commandment we have from him: that the one who loves God should love his brother also.

And this commandment have we from him, That he who loveth God love his brother also.

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