Latin iustitia aliena, a phrase popularized by Martin Luther to describe the righteousness by which a Christian stands before God. "Alien" here means "foreign, belonging to another, not native to oneself" — not in the sense of extraterrestrial. Luther drew the term from his reading of Paul, especially Philippians 3:9 — "not having a righteousness of my own that comes from the law, but that which comes through faith in Christ." The believer's righteousness is not generated within him; it is received from outside, from Christ, and reckoned to him by imputation.
Alien righteousness is the Reformation's theological revolution in a phrase. Before Luther, Western theology had implicitly collapsed the righteousness that justifies into the righteousness progressively infused into the soul — meaning acceptance by God was tied to measurable personal holiness. Luther's retrieval: Christians are accepted on account of Christ's righteousness, which is always external to them (alien) and always given, never earned. This doctrine has three stabilizing effects. (1) Assurance — you don't have to inspect your own holiness to know you are accepted; you inspect Christ's finished work. Your status before God is as secure as Christ's. (2) Humility — there is no room for boasting, because you bring nothing of your own to the transaction. Even your faith is a gift (Ephesians 2:8-9). (3) Motivation — counterintuitively, knowing your acceptance is secure frees obedience rather than paralyzing it. The Christian does not work for justification; he works from justification, gratefully. Charges of antinomianism (lawlessness) against this doctrine are usually misdirected: gratitude is a stronger motivator than terror. Alien righteousness is not a license to sin; it is the ground on which any genuine holiness can be built.