I’m writing to the Christian who feels depressed because they repeatedly fail to obey God’s law. It’s not that you want to disobey, but instead you say with Paul, “I do not do what I want, but I do the very thing I hate” (Rom. 7:15). You love God but you don’t love him perfectly. In fact, sometimes you even question if your love is sincere. “If I really loved God, would I have responded like that!?” You long to overcome certain sins in your life, but you question whether or not you’ll ever be able to. I’m writing to you.
In the Old Testament, God gave his sinful people a promise: “The LORD your God will circumcise your heart and the heart of your offspring, so that you will love the LORD your God with all your heart and with all your soul, that you may live.” (Deut. 30:6)
If you’re familiar with the history of God’s people, you know they struggled repeatedly with obedience to God. They broke the first covenant he made with them, the Mosaic Covenant, so God promised to make a new covenant. This verse in Deuteronomy 30 is referring to that covenant (it’s also described in places like Jeremiah 31:31-34). In this new covenant, God was going to forgive his people all their sins and write his law of love upon their hearts. When Jesus died on the cross, he set in motion that new covenant (Luke 22:20; 1 Cor. 11:25). If you’ve placed your trust in Jesus Christ, you are spiritually alive, and the new heart promised in the new covenant is beating within you.
But I don’t love the LORD with all my heart and with all my soul like Deuteronomy 30:6 promised. I wish I did! You need to understand that right now–this side of glory–your love for God will falter due to sin. No one perfectly keeps the law of God, which is precisely why the Father sent Jesus into the world. Jesus obeyed the law perfectly in the place of law breakers. You can stand confident before the Father because of Christ’s obedience, knowing that the once-bitter law no longer has the power to threaten you. Freed from its condemnation, you can now begin to fulfill God’s law of love by the Spirit, in a way that was previously impossible.
The Protestant Reformer Martin Luther talked about the two ways God’s law is presently fulfilled in the faithful:
Therefore, since we cannot fulfill the Law because of sin reigning in our flesh and holding it captive, Christ came and killed that sin by sin–that is, by the sacrifice made for sin–that in this way the righteousness required by the Law might be fulfilled in us: first, by way of imputation, and then formally as well [that is, in reality]–yet this is not of ourselves [cf. Eph. 2:8] but by the grace of God, who sent His Son in the flesh. He gives the Spirit to those who believe these things so that they begin to hate sin sincerely; to recognize the immense, incomprehensible, and ineffable gift; to give thanks to God for it; to love, worship, and call on God; and to expect everything from Him. For if he gave up his son — gave him up for sins — he will surely also give us all things with him. [Rom. 8:32]
Why do you hate the sin you struggle with? Because God has sent his Holy Spirit to live in you. Indeed, there would be no struggle at all if it wasn’t for the presence of the Holy Spirit in your life. According to Luther, the law is perfectly fulfilled in you first by imputation. That is, God credited you with the perfect, righteous life of Jesus by faith, so that you could be justified despite your imperfect obedience.
But there’s more to the story than just this. Luther goes on to say that now, formally, the law begins to be fulfilled in us by the power of the Holy Spirit (albeit imperfectly). This is the second way in which the law is fulfilled by believers, and it’s what Paul referred to in Romans 8:3-4, “For God has done what the law, weakened by the flesh, could not do. By sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh, in order that the righteous requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit.”
God has counted you righteous in justification, and by the Spirit he is making you righteous day by day, causing you to hate sin, and giving you the desire to love God even better than you do now. Jesus rendered the perfect obedience that secured your adoption into God’s family, and because of the work of Jesus and the Holy Spirit, one day you too will give God the perfect love you long to give him.
The apostle John wrote, “Beloved, we are God’s children now, and what we will be has not yet appeared; but we know that when he appears we shall be like him, because we shall see him as he is. And everyone who thus hopes in him purifies himself as he is pure.” (1 John 3:2-3)
One day the battle against sin will be over. When Christ appears, our lowly bodies will be transformed (Phil. 3:21), and we will begin to love God and one another perfectly. Right now your obedience may feel weak, but in the age to come it will be perfect, just like Christ’s. Again, Luther wrote, “In the life to come it will not be necessary to admonish us to love God. But then we will truly and perfectly do what Christ did here. Then you will not say, ‘I should love the Father,’ but ‘I love the Father’ and ‘The very thing I have been commanded is what I do.'”
 Luther, Martin First Antinomian Disputation; See Argument 14
 Ibid; See Argument 7