Sanctification
The Holy Spirit's work of making a justified sinner actually holy over time. Lutherans insist it is never the ground of salvation and never optional either: good works are necessary, but not necessary for justification. The distinction sounds like hair-splitting until you are the one wondering whether you have done enough, at which point it turns out to be the whole question.
In Doctrine — What Lutherans teach, and where it differs from neighbouring traditions.