1 John 2:5

5But 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 keephis 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)

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.