Q&A: Why Obey God's Law?

Question

In Future Grace, John Piper refutes the notion that gratitude is the proper motivation for holy living. Instead, Piper claims that faith in God's future grace should compel us to sanctification. Is this the only legitimate motivation for holiness?

Answer

There are many valid motivations for obeying God's law.

One valid reason to obey God is indeed gratitude or thanks for the grace he has already shown us (e.g. Col. 2:6-7).
Another reason to live righteously is that it pleases God. If we love him, we will desire to please and to glorify him. This is quite similar to the way we behave in our other relationships. For example, we treat our family members well because we love them, we treat our friends well because we want them to be happy, etc. As Jesus said in John 14:15, "If you love me, you will keep my commandments" (cf. John 14:21-24; 15:14); and in John 15:8, "My Father is glorified by this, that you bear much fruit, and so prove to be my disciples." Part of the idea behind these verses is that real love expresses itself in ways intended to please the one who is loved -- just as Jesus himself did (John 14:31). Those who do not really love God are not saved.
We also enjoy present blessings as a result of obedience, such as sanctification itself (e.g. Rom. 6:19), the enormous host of benefits mentioned in Psalm 119, and security in our salvation (2 Pet. 1:10). Psalm 1:1-3; Genesis 22:18; Deuteronomy 28:1ff. are other texts which teach this idea.
When we obey God, we store up future rewards for ourselves (e.g. Matt. 6:2-6; Luke 6:35; 12:33; Rom. 2:6-7; 1 Cor. 3:8,14; 9:17-18; 2 Cor. 5:9-10; Jam. 1:12; 2 John 8; Rev. 2-3; 22:12), and also for others (e.g. 1 Tim. 4:16; 1 Pet. 3:1-2).
Related to this, another valid reason is the threat of eternal judgment (e.g. Heb. 12:14) -- not that true believers can ever be lost, but some of us are self-deceived, being actually unsaved. Hell will be worse for those who perish who have been more wicked.
And even for those who are headed for heaven, obedience helps avoid God's temporal discipline (e.g. Heb. 12; Rev. 3:19). Sometimes this discipline can be quite harsh, as when God killed David's newborn child (2 Sam. 12:13-20).
A reason Paul frequently mentions is that we are new people with new characters, so that our lives ought to be in keeping with who we now are (e.g. Rom. 6).
Closely related to the foregoing reason is that the Holy Spirit compels believers to live righteously. He changes our hearts so that we want to obey God, and works in us to make us desire to do good things, and actually to perform them (Phil. 2:13).
Then too, we ought to be obedient because of our concern for our fellow man - our obedience is a witness of the gospel to them (e.g. Phil. 2:15; 1 Pet. 2:12,15), and of God's glory (Matt. 5:14-16; Rom. 2:24).
Additionally, because we are saved, we enjoy obeying God and are distressed by our disobedience (Ps. 51; Rom. 7:14-25).