If God does everything, why am I still told to fight sin?

Because Scripture never sets God’s working and your working against each other. It grounds the second in the first.

Paul puts both halves in one sentence and will not surrender either: work out your own salvation with fear and trembling, for it is God who works in you, both to will and to work. He does the same thing to himself — he labored harder than all the apostles, though it was not him but the grace of God. Not “God worked, so I rested.” Not “I worked, and then God helped.” The working is real and the grace is the cause of it.

The confusion usually comes from a quietist assumption that has drifted into ordinary piety: that any effort on your part must be subtracted from God’s glory, so the holier path is to stop. But that assumption has to explain away most of the New Testament, which is full of imperatives — put to death what is earthly in you, flee, cut it off, put on, keep, watch, stand. Those are not addressed to unbelievers as terms of entry. They are addressed to the baptized as the shape of a life the Spirit has already begun.

The distinction that keeps this straight is the old Lutheran one between justification and sanctification. Your standing before God is finished, imputed, and entirely outside you; you contribute nothing to it and cannot improve it. Your life in the world is a battlefield, and you will be on it until you die. Collapse the two and you get either a Christian frantically trying to earn what he already has, or a Christian who has stopped fighting because he thinks fighting would insult grace. Both are miserable, and neither is what the Confessions describe.

So what do you do with the fact that you keep losing?

Romans 7 is the honest answer. The man there is not a beginner; the struggle intensifies as the Spirit arrives, because before that there was nothing in you to fight against sin at all. You are simultaneously righteous and a sinner — not fifty percent of each, but wholly both at once, righteous in Christ and sinful in yourself, right up to the last day.

The presence of the fight is not evidence that grace failed. A corpse does not struggle. The struggle is a vital sign.