On October 31, 1517, Martin Luther nailed his 95 Theses concerning clerical abuses and indulgences on the church door at Wittenberg. This famous event is often considered that launching point for the Protestant Reformation.

The chief concern for Luther and the other reformers was the doctrine of justification. It was, to use Calvin’s language, the “main hinge on which religion turns.” And the doctrine of justification is no less important today than it was 500 years ago.

There are five key concepts every Protestant should grasp if they are to understanding the reformer’s (and the Bible’s) doctrine of justification.

First, the Christian is simul iustus et peccator. This is Martin Luther’s famous Latin phrase which means “At the same time, justified and a sinner.” The Catechism powerfully reminds us that even though we are right with God, we still violate his commands, feel the sting of conscience, and battle against indwelling sin. On this side of the consummation, we will always be sinning saints, righteous wretches, and on occasion even justified jerks. God does not acquit us of our guilt based upon our works, but because we trust “him who justifies the ungodly” (Rom. 4:5).

Second, our right standing with God is based on an alien righteousness. Alien doesn’t refer to an E.T. spirituality. It means we are justified because of a righteousness that is not our own. I am not right with God because of my righteousness, but because “the perfect satisfaction, righteousness, and holiness of Christ” has been credited to me. “Nothing in my hands I bring, simply to thy cross I cling; naked, come to thee for dress; helpless, look to thee for grace; foul, I to the Fountain fly; wash me, Savior, or I die” wrote August Toplady in the old hymn. We contribute nothing to our salvation. The name by which every Christian must be called is “The Lord is our righteousness” (Jer. 23:6).

Third, the righteousness of Christ is ours by imputation, not by impartation. That is to say, we are not made holy, or infused with goodness as if we possessed it in ourselves, but rather Christ’s righteousness is credited to our account.

Fourth, we are justified by faith alone. The Catholic Church acknowledged that the Christian was saved by faith; it was the alone part they wouldn’t allow. In fact, the Council of Trent from the 16th century Catholic counter-reformation declared anathema those who believe in either justification by imputation or justification by faith alone. But evangelical faith has always held that “all I need to do is accept the gift of God with a believing heart.” True, justifying faith must show itself in good works. That’s what James 2 is all about. But these works serve as corroborating evidence, not as the ground of our justification. We are justified by faith without deeds of the law (Rom. 3:28; Titus 3:5). The gospel is “believe on the Lord Jesus Christ and you shall be saved” (Acts 16:30-31), not “believe on the Lord Jesus Christ and cooperate with transforming grace and you shall be saved.” There is nothing we contribute to our salvation but our sin, no merit we bring but Christ’s, and nothing necessary for justification except for faith alone.

Finally, with all this talk about the necessity of faith, the Catechism explains that faith is only an instrumental cause in our salvation. In other words, faith is not what God finds acceptable in us. In fact, strictly speaking, faith itself does not justify. Faith is only the instrument by which we embrace Christ, have communion with him, and share in all his benefits. It is the object of our faith that matters. If you venture out on to a frozen pond, it isn’t your faith that keeps you from crashing into the water. True, it takes faith to step onto the pond, but it’s the object of your faith, the twelve inches of ice, that keeps you safe. Believe in Christ with all your heart, but don’t put your faith in your faith. Your experience of trusting Christ will ebb and flow. So be sure to rest in Jesus Christ and not your faith in him. He alone is the one who died for our sakes and was raised for our justification. Believe this, and you too will be saved.