But whoso keepeth He now defines what a true keeping of God's law is, even to love God. This passage is, I think, incorrectly explained by those who understand that they please the true God who keep his word. Rather take this as its meaning, "to love God in sincerity of heart, is to keep his commandments." For he intended, as I have before reminded you, briefly to shew what God requires from us, and what is the holiness of the faithful. Moses also said the same thing, when he stated the sum of the law. "Now, O Israel, what does the Lord require of thee, but to fear and love him, and to walk in his precepts?" (Deuteronomy 10:12.) And again he says, "Choose life, even to love the Lord thy God, to serve him and to cleave to him." (Deuteronomy 30:19, 20) For the law, which is spiritual, does not command only external works, but enjoins this especially, to love God with the whole heart. That no mention is here made of what is due to men, ought not to be viewed as unreasonable; for brotherly love flows immediately from the love of God, as we shall hereafter see. Whosoever, then, desires that his life should be approved by God, must have all his doings directed to this end. If any one objects and says, that no one has ever been found who loved God thus perfectly; to this I reply, that it is sufficient, provided every one aspired to this perfection according to the measure of grace given unto him. In the meantime, the definition is, that the perfect love of God is the complete keeping of his law. To make progress in this as in knowledge, is what we ought to do. Hereby know we that we are in him He refers to that fruit of the gospel which he had mentioned, even fellowship with the Father and the Son; and he thus confirms the former sentence, by stating what follows, as a consequence. For if it be the end of the gospel to hold communion with God, and no communion can be without love, then no one makes a real progress in faith except he who cleaves from the heart to God.

But whoso keepeth his word - That is, what he has spoken or commanded, The term "word" here will include all that he has made known to us as his will in regard to our conduct.
In him verily is the love of God perfected - He professes to have the love of God in his heart, and that love receives its completion or filling up by obedience to the will of God. That obedience is the proper carrying out, or the exponent of the love which exists in the heart. Love to the Saviour would be defective without that, for it is never complete without obedience. If this be the true interpretation, then the passage does not make any affirmation about sinless perfection, but it only affirms that if true love exists in the heart, it will be carried out in the life; or that love and obedience are parts of the same thing; that one will be manifested by the other; and that where obedience exists, it is the completion or perfecting of love. Besides, the apostle does not say that either the love or the obedience would be in themselves absolutely perfect; but he says that one cannot fully develop itself without the other.
Hereby know we that we are in him - That is, by having in fact such love as shall insure obedience. To be in him, is to be united to him; to be his friends. Compare the John 6:56 note; Romans 13:14 note.

But whoso keepeth his word - Conscientiously observes his doctrine, the spirit and letter of the religion of Christ.
Is the love of God perfected - The design of God's love in sending Jesus Christ into the world to die for the sin of man τετελειωται, is accomplished, in that man who receives the doctrine, and applies for the salvation provided for him. This seems to be the meaning of the apostle.
That we are in him - That we have entered into his spirit and views, received his salvation, have been enabled to walk in the light, and have communion with him by the Holy Spirit.

(4) But whoso keepeth his word, in him verily is the (f) love of God perfected: hereby know we that we are in (g) him.
(4) He that keeps God's commandments loves God indeed: He that loves God, is in God, or is joined together with God. Therefore he that keeps his commandments is in him.
(f) Wherewith we love God.
(g) He means our union with Christ.

But whoso keepeth his word,.... Either the word of the Gospel, and the truths of it, who receives it in love, cordially embraces and retains it, and will by no means part with it, but holds it fast, and stands fast in it; or the precepts and ordinances of the word, who loves these, and esteems them above fine gold, and concerning all things to be right, and observes them as they should be:
in him verily is the love of God perfected: not the love wherewith God loves him, for that is perfect in himself, and admits of no degrees, and cannot be more or less in his heart, and is entirely independent of the obedience of men, or any works of theirs; it is true indeed the manifestations of this love to the saints are imperfect, and may be more and greater, and greater manifestations of love are promised to such that love Christ, and keep his commandments, John 14:21; but here it is to be understood not actively, but passively, of the love wherewith God is loved by his people; and intends not the absolute perfection of it in them, in whom it often waxes cold, and is left, or the fervour of it abated, but the sincerity and reality of it; for by keeping the word of God, both his truths and his ordinances, it is clearly seen that their love to him is without dissimulation, and is not in tongue only, but in deed and in truth:, now it is not the keeping of the word of God that causes this love, or makes it perfect or sincere, for it is a fruit of the Spirit, and is owing to the grace of God; but love, on the other hand, is the cause of keeping of the word; and the latter being a consequent and an effect of the former, is the evidence of it, of the truth and sincerity of it:
hereby know we that we are in him: in Christ, not merely nominally, or by profession, as all that name the name of Christ, and are in a Gospel church state, may be said to be; but really, first secretly, through the love of Christ, the election of God, and the covenant of grace, and then openly, in conversion and the effectual calling, through believing in Christ, when the saints appear to be in him as branches in the vine; and which is known by their fruits, as here, by keeping the word, and doing the commandments of Christ, which do not put a man into Christ, but only show that he is there; for a man's being in Christ is owing to the grace of God; this is the first thing done in grace, 1-Corinthians 1:30.

Not merely repeating the proposition, 1-John 2:3, or asserting the merely opposite alternative to 1-John 2:4, but expanding the "know Him" of 1-John 2:3, into "in Him, verily (not as a matter of vain boasting) is the love of (that is towards) God perfected," and "we are in Him." Love here answers to knowledge in 1-John 2:3. In proportion as we love God, in that same proportion we know Him, and vice versa, until our love and knowledge shall attain their full maturity of perfection.
his word--His word is one (see on 1-John 1:5), and comprises His "commandments," which are many (1-John 2:3).
hereby--in our progressing towards this ideal of perfected love and obedience. There is a gradation: 1-John 2:3, "know Him"; 1-John 2:5, "we are in Him"; 1-John 2:6, "abideth in Him"; respectively, knowledge, fellowship, abiding constancy. [BENGEL].

But whoso keepeth his word - His commandments. Verily in him the love of God - Reconciled to us through Christ. Is perfected - Is perfectly known. Hereby - By our keeping his word. We know that we are in him - So is the tree known by its fruits. To "know him," to be "in him," to "abide in him," are nearly synonymous terms; only with a gradation, - knowledge, communion, constancy.