74 The καί (kai) is epexegetical/explanatory (or perhaps resumptive) of the commandment(s) mentioned in the preceding verse.
75 This verse begins with the phrase καὶ αὔτη ἐστίν (kai hautē estin; cf. the similar phrase in 3:11 and 1:5), which is explained by the following ἵνα (hina) clause, “that we believe in the name of his Son Jesus Christ.” The ἵνα thus introduces a clause which is (1) epexegetical (explanatory) or (2) appositional. By analogy the similar phrase in 3:11 is also followed by an epexegetical ἵνα clause and the phrase in 1:5 by an epexegetical ὅτι (hoti) clause. His commandment refers to what follows—the commandment from God is to believe in his Son, Jesus Christ, and to love one another.
76 The author of 1 John repeatedly attributes the commandments given to believers as given by God the Father, even though in John 13:34–35 it was Jesus who gave the commandment to love one another. 2 John 4–5 also attributes the commandment to love one another directly to the Father. Thus it is clear that God the Father is the subject of the verb gave here in 3:23.

In other versions

And this is his commandment, that we believe in the name of his Son Jesus Christ and love one another, just as he has commanded us.

And this is his command: to believe in the name of his Son, Jesus Christ, and to love one another as he commanded us.

Now this is his command: that we believe in the name of his Son, Jesus Christ, and love one another as he commanded us.

And this is his commandment: We must believe in the name of his Son, Jesus Christ, and love one another, just as he commanded us.

And this is his commandment: that we believe in the name of his Son Jesus Christ and love one another, just as he commanded us.

And this is his commandment, That we should believe on the name of his Son Jesus Christ, and love one another, as he gave us commandment.

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