Faith alone (sola fide) is one of the Reformation’s key slogans. Those descended from the Reformation may not agree on everything. But they all hold that God justifies the ungodly by faith alone apart from works of the law. Lutherans, Calvinists, and other Reformation Protestants can at least agree on this. Or at least they’re supposed to. Faith alone is much more than just a slogan; it is the key doctrinal pillar of all who claim to be Protestant.

But in practice, faith alone is not always so simple a doctrine to maintain. The temptation to obscure the meaning of justification by faith is a persistent one. This is because faith alone threatens the whole possibility of contributing anything to our salvation. Sinners are convinced that there must be something left for us to do. Pleasing God has to depend on us in some way. This is the essence of what Gerhard Forde used to call the “legal scheme.” Sinners are captive to sin’s most deep-seated lie of all: that God’s law is a ladder we use to climb upwards into his presence and favor, instead of the hammer he uses to break all our self-righteous illusions.

Repentance is one of those places in Christian doctrine where the legal scheme can slip back in. The idea of salvation as a two-way exchange between God and us can subtly undermine justification by faith alone if repentance becomes our side of the bargain. We might affirm that justification is by grace alone and received by faith alone. But bookending justification is the repentance that comes first and the sanctification that comes after. Here, we might suspect, is where we can make our contribution. God needs our repentance first and our sanctification afterward. Then he’ll be pleased, and salvation will be a joint effort.

The scriptural truth is that both repentance and sanctification are gifts of God. He works them in us with his word. Sanctification as a divine work in us would require its own treatment. But the Bible makes clear how repentance is an activity of God’s word most clearly in Peter’s famous Pentecost sermon.

Here, Peter delivers both the law and the gospel. The law reveals the bad news of our sin. Sin manifests itself most supremely in our indignation at needing a savior. The original sin is making ourselves into gods. And when God arrived to forgive this sin, we sinners rejected him and put him to death. This is the law’s word of judgment. But the good news is that the risen Christ bears away all our sin and delivers the benefits of his death to his enemies – especially the gift of his Holy Spirit.

So what to do if Peter’s sermon is all true? His words might imply we must do something first in the form actively repenting and then passively being baptized. Perhaps repentance is something that we do first. Then, God will give us the forgiveness of sins and the gift of the Spirit. But don’t forget this question – “what shall we do”– is preceded by Luke’s commentary that those listening were “cut to the heart” by Peter’s words (2:37). Repentance means being cut down by the law’s declaration of judgment. It’s not an activity we do to prepare for grace, but a point of despair worked by God himself.

The proper manifestation of repentance is not knowing what to do next, like Peter’s hearers. This is the law’s work. It leaves us no alternatives. It destroys any possibility of escape and eliminates the idea that we will contribute merit or worthiness to our salvation. But the law cannot save us, even while it is necessary to bring about our repentance. This is where the gospel promise delivers us from the despair and judgment of the law.

Baptism’s work is the most important demonstration of what the gospel gives. Instead of full awareness of our sin, baptism gives us confidence in the righteousness of Christ given to us. This is what it means to have your sin forgiven. And instead of being dead in sin, baptism enlivens us with the Holy Spirit that Christ sends with his word in the water.

Repentance is God’s work as much as justification is. It’s something he does with his word, as Peter’s sermon demonstrates. The comfort in all this for the Christian is that repentance isn’t something I have to perfect before I can receive salvation. Repentance is actually a work of God with his word. And any good sermon – like Peter’s – will leave you with no more repenting to do but only a gospel promise to believe.