What does it mean to be justified by faith alone, but not by a faith that is alone?
This saying is attributed to Martin Luther, and both halves of it are true. But usually folks lean one way or another on this, causing no end of theological controversy. In one camp are the “faith alone” folks and in the other camp are the “faith isn’t alone” folks.
God justifies us, declares us righteous in His sight, without considering our works at all. Like the thief on the cross, or Mary Magdalene, you never have to worry that awful things you have done, or good things you’ve not done, jeopardize your salvation, as long as you are trusting Jesus Christ to atone for your sins at the cross. God doesn’t care how much damage you’ve done by your sins, if you are truly trusting His provided sacrifice to pay for your sins. Works do not enter the equation in any way. To call your faithfulness your faith in this justification equation is dangerous and starts to sound like justification by works. In this regard, we want to keep a strict separation between our trust, however faltering, and our life which remains sinful. Do not put any trust or hope in your life of integrity. Any obedience you have is downstream from God justifying and changing you before.
Faith isn’t alone
The way we and others see our faith is by our works. This is the point in James 2:14 and following. We show our faith by our works, but we don’t earn our justification by our works. We should be diligent to make our calling and election sure, to do good works which God created us before to do. If we say we believe, but never act like a Christian, it’s a hollow profession that will not justify. We can reach a point where we are so unfaithful in the present, that it discredits our profession of faith before God. God makes this determination at the end of your life, as shown in Matthew 7:21-23. You can’t rely on a profession of faith in the past or present to save you, if your heart is far from God and you are living as you want without regard for God.
Suspicion abounds; affirm the truth
Both of the above paragraphs are true. But some will emphasize the first and suspect the “faith isn’t alone” camp of legalism. Others stress the second and suspect the “faith alone” camp of antinomianism. If you are worried about this in someone, just check that they believe both sides. If they do, don’t sweat it too much that they emphasize the other camp. Teachers should hit both of these about equally in their teaching.