1 John 3:23

And this is his commandment
Having mentioned the keeping of the commandments of God, the apostle proceeds to show what they are; that they are faith in Christ, and love to one another; which two are reduced to one, because they are inseparable; where the one is, the other is; faith works by love.

That we should believe on the name of his Son Jesus Christ;
Christ is the object of faith, and he is no, as he is Jesus, a Saviour; faith deals with him as such, and will have no other Saviour but he: and now to believe in him, is not merely to believe that he is the Son of God, the true Messiah, the Saviour of the world, that he is come in the flesh, has suffered, and died, and rose again from the dead, is ascended into heaven, and is set down at the right hand of God, makes intercession for his people, and will come again to judge the quick and dead; but it is to go forth in special and spiritual acts upon him, such as looking at him, coming to him, venturing on him, trusting in him for life and salvation, committing all into his hands, and expecting all from him. And this is called a "commandment", and comes under the notion of one; not that it is properly a law, or belongs to the law; for faith in Christ Jesus is a fruit of electing grace, and a blessing of the covenant of grace; it is the free gift of God, and the operation of his Spirit, and is peculiar to the elect of God, and sheep of Christ; and so cannot belong to the law of works; but, as the Hebrew words, (hrwt) , and (hwum) , both signify any doctrine, and instruction in general; see ( Psalms 19:7 Psalms 19:8 ) ; so the word here used designs an evangelical doctrine, a divine instruction of our Lord Jesus Christ, in the Gospel, which declares that he that believes in Christ shall be saved; and so the word is used for a doctrine in this epistle, ( 1 John 2:7 ) ; and that of the next command or doctrine, which follows,

and love one another as he gave us commandment;
that is, as Christ taught and instructed his disciples, ( John 13:34 ) ( 15:17 ) .