Christianity has argued about the mechanism of salvation more intensively than any other tradition, and the sixteenth-century division of Western Christendom turned on it.
The instrument. Whether a person is justified by faith alone, the Reformation's sola fide, resting on Romans and Galatians, or by faith working through love, with grace, the sacraments and human cooperation. The Council of Trent's decree on justification (1547) condemned the doctrine that faith alone justifies while equally condemning the view that anyone is saved without grace, a symmetry often overlooked in polemical accounts of it. The Lutheran-Catholic Joint Declaration on the Doctrine of Justification (1999) stated a shared understanding sufficient, in its signatories' judgement, to lift the sixteenth-century mutual condemnations on this point; a number of Lutheran theologians disputed that the substance had been resolved rather than the language softened.
The scope. Whether Christ died for all, as Catholic and Arminian theology holds, or for the elect alone, as strict Calvinism's particular redemption holds; and whether grace can be resisted. The Synod of Dort (1618-19) defined the Reformed position against the Remonstrants, producing the five points later summarised by the acronym TULIP.
The mechanism of the atonement has never been dogmatically defined in any communion, and several models coexist. Ransom and Christus Victor dominate the patristic period, presenting the cross as a victory over death and the powers. Anselm's Cur Deus Homo (c. 1098) replaced ransom with satisfaction owed to God's honour. Abelard emphasised the moral effect on the beholder. Penal substitution, articulated in the Reformation and dominant in much modern evangelicalism, holds that Christ bore the punishment due to sin; it is criticised by others as importing a juridical scheme foreign to the texts and, in its harsher popular forms, as morally objectionable. Eastern Christianity has generally been less interested in the question of mechanism, framing salvation as theosis, participation in the divine life, rather than as the settling of a debt.
The extent. Whether those outside the church can be saved. Extra ecclesiam nulla salus was stated severely in the medieval West, at its sharpest in the Council of Florence. The Second Vatican Council's Lumen Gentium and Nostra Aetate affirmed that those who through no fault of their own do not know the gospel, yet seek God and follow conscience, may attain salvation. Karl Rahner pressed the inclusivist logic further with the phrase "anonymous Christianity", which drew objection from exclusivists as emptying the gospel of urgency and from pluralists as annexing other traditions without their consent.