It can be easy to think of theology as a pile of fruit, whereby you can pick the ones you like and leave the ones you don't. However, theology, in general, is much more organic, in that it forms complete systems. That is, we can't sensibly cherrypick doctrines from competing systems without breaking them and running into inconsistencies.

Still, if I had to point to a single doctrine or article of faith that sets Lutheranism apart from other theological systems, it would have to be the Lutheran doctrine on Justification. This is the chief article of faith as it concerns the purity of the Gospel.

Sola Gratia

By Grace Alone

Justification denotes being made free from sin and eternal punishment of sins and declared righteous before God. We cannot be justified before God by our own strength. That is, we cannot obtain righteousness and the forgiveness of sin before God by our own merits or works. Rather, we are justified before God, by grace, for Christ's sake, through faith in the person and works of Jesus Christ. If we believe that Christ suffered for us, sins are remitted unto us for Christ's sake, and righteousness and eternal life are bestowed on us. For, God accounts and imputes this faith to us as righteousness before Himself, as Paul says in Romans 3, 4.

From beginning to end, justification, or salvation, is the work of God. We are justified by faith alone, apart from works. We could say that justification consists of (1) God's grace, (2) the person and works of Christ, and (3) faith, which apprehends God's promise of the Gospel.

God, being rich in mercy, because of the great love with which He loved us, even when we were dead in sin, made us alive together with Christ. By grace we have been saved. (Ephesians 2)

By the obedience of Christ - his fulfilling of the Law, his passion, death and resurrection - we are made righteous before God. That is, we are declared righteous in God's sight in that we are clothed in Christ's righteousness. (Isaiah 61:10)

Justifying faith is not a simple acknowledgement of the existence of God (James 2:19), nor a mere historical knowledge of Jesus Christ (Matthew 7:21). Justifying faith is a living faith, a gift from God, worked by the Holy Spirit, by which, in the Gospel, we correctly recognise Christ as our Lord and Redeemer, and trust in him and his work. It's a divinely given trust in God and His promise of salvation through Christ.

We do not justify ourselves or supplement justification with good works. Neither does good works increase or sustain justification. Having been born of the Holy Spirit, we cannot finish by works of the flesh. (Galatians 3) It is God who saves and upholds us, by His own goodness, kindness, love and grace. Justification, or salvation, is entirely from God and not from any human work, will or decision.

John 1:11-12: "He came to his own, and his own people did not receive him. But to all who did receive him, who believed in his name, he gave the right to become children of God, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God."