I have found this a helpful word on the relationship of Christ’s finished work in salvation and our needed work in repentance. I hope it helps you too.

Paul declared, “I have declared to both Jews and Greeks that they must turn to God in repentance and have faith in our Lord Jesus” (Acts 20:21).

From this verse, Sam Waldron shows that repentence is a necessary activity for someone to be saved. He explains…

How, then, do I dare to speak of doing anything to be saved? Good question! The simple answer is that both Acts 20:21 and many parallel texts require me so to speak. Here in Acts 20:21 the Word of God speaks of repentance and faith as the things the gospel requires of us if we are to be saved. These are not things God in Christ does for us. They are things we must do.

Now clearly we have a difficulty, don’t we? Christ’s work is finished. We are not saved by “deeds which we have done in righteousness.” Yet there is something God calls on us to do. There are activities required – things to be done – in order to be saved by the gospel. To obscure this plain fact is to darken the way of salvation to the eyes of men.

Clearly, we must distinguish two kinds of doing. Perhaps I can illustrate it for you. One of my fellow pastors much prefers driving to flying. Suppose he had problems with his van driving from Kentucky to California – serious problems! Somewhere in Arizona the van is fixed.

Now listen to what the mechanic says when he hands my fellow pastor the key to his van: “The job is done. There is nothing left to do. All you have to do is just start her up.” Do you see how silly it would be for my fellow pastor to say, “If there is nothing left to do, why do I need this key? Why do I have to start her up? Why do I have to do anything?” Everybody knows that the mechanic is talking about two completely different kinds of doing. Fixing the van and turning the key are not the same kind of doing at all.

There are activities required of us by the gospel. They are on a completely different level than the work of Christ. They merit nothing from God. They are themselves gifts of grace. They are the opposite of human achievements that contribute to Christ’s finished work, but they are activities or conditions required of men in the gospel.

It is not a doing that creates salvation, but a doing that receives it.

Taken from Two Things You Must Do to Be Saved, p. 11-12.