God requires and rewards good works (2 Corinthians 5:10). But we must be careful to remember that even our best works cannot merit the pardon of our sin (Romans 3:20), or earn us eternal life (Ephesians 2:8,9). There are two basic reasons that we cannot merit forgiveness or earn eternal life by our works.
The first reason is that there is a great disparity between the quality of our good works and the quality of forgiveness and eternal life. Supposing, for a moment, that we always (and only) did our very best works; we would still be exchanging merely finite and temporal labor for infinite and eternal rewards. We can illustrate this principle by currency exchanges among the nations on earth. At the time of this writing, 1 US dollar is worth 41,600 Iranian rials. This means that it would require about 172,640 Iranian rials to buy just a cup of coffee in America. The exchange rate between the United States and Iran is not very favorable to Iran. The exchange rate between heaven and any place on earth is even less favorable. The glories of heaven are so incomparably greater than our labors on earth, that we could never earn anything in heaven, even if we only did good, and had infinite time to earn it. But, of course, we don’t only do good, and we don’t have infinite time to do it. In fact, we are actually already in debt to heaven when we begin, we have but a moment to labor on the earth, and we don’t always do good.
The second reason that our good works cannot merit forgiveness or earn eternal life is the great distance between us and God. God is the infinite, eternal, almighty Creator. We are His creatures. Even if we never sinned, we would still owe our very existence to God. Suppose, for a moment, that Adam had perfectly obeyed God and had never eaten the forbidden fruit. Would we say that God would have been in Adam’s debt? Of course not! If he had obeyed God, Adam would have only done the bare minimum of what he what he was required to do. So too with us. We only have life because God gave it to us. If we use that life to obey him, we are only rendering to Him that which is due, using that which He gave us. But remember, like Adam, we have not perfectly obeyed God. In fact, we have often sinned against Him. Meanwhile, the good works (when they are good) that we do (when we do them) are done by the life that God gave us. God doesn’t owe us anything. We don’t provide God with anything that He hasn’t already given us. We can’t make God our debtor. This is why Jesus said, “When ye shall have done all those things which are commanded you, say, ‘We are unprofitable servants: we have done that which was our duty to do.’” (Luke 17:10) When we do good works, we are only doing our duty.
As creatures and servants, we owe everything to God, and provide nothing for God. As sinners, we not only fail to give God what we already owe Him, we also put ourselves further in His debt. Therefore, we cannot merit His forgiveness or eternal life. That’s why we need the merit of Christ’s sacrifice for our pardon and the gift of eternal life.
Since we can’t earn God’s favor by doing good, someone might conclude that we ought not even try and do good. But that would be the wrong conclusion. The right conclusion is that we should do good and be humble.