Why did Luther add the word 'alone' to Romans 3:28?

At 3:28 Luther introduced the adv. “only” into his translation of Romans (1522), “alleyn durch den Glauben” (WAusg 7.38); cf. Aus der Bibel 1546, “alleine durch den Glauben” (WAusg, DB 7.39); also 7.3-27 (Pref. to the Epistle). See further his Sendbrief vom Dolmetschen, of 8 Sept. 1530 (WAusg 30.2 [1909], 627-49; “On Translating: An Open Letter” [LuthW 35.175-202]). Although “alleyn/alleine” finds no corresponding adverb in the Greek text, two of the points that Luther made in his defense of the added adverb were that it was demanded by the context and that sola was used in the theological tradition before him.

Origen, Commentarius in Ep. ad Romanos, cap. 3 (PG 14.952).

Hilary, Commentarius in Matthaeum 8:6 (PL 9.961).

Basil, Hom. de humilitate 20.3 (PG 31.529C).

Ambrosiaster, In Ep. ad Romanos 3.24 (CSEL 81.1.119): “sola fide justificati sunt dono Dei,” through faith alone they have been justified by a gift of God; 4.5 (CSEL 81.1.130).

John Chrysostom, Hom. in Ep. ad Titum 3.3 (PG 62.679 [not in Greek text]).

Cyril of Alexandria, In Joannis Evangelium 10.15.7 (PG 74.368 [but alludes to Jas 2:19]).

Bernard, In Canticum serm. 22.8 (PL 183.881): “solam justificatur per fidem,” is justified by faith alone.

Theophylact, Expositio in ep. ad Galatas 3.12-13 (PG 124.988).

To these eight Lyonnet added two others (Quaestiones, 114-18):

Theodoret, Affectionum curatio 7 (PG 93.100; ed. J. Raeder [Teubner], 189.20-24).

Thomas Aquinas, Expositio in Ep. I ad Timotheum cap. 1, lect. 3 (Parma ed., 13.588): “Non est ergo in eis [moralibus et caeremonialibus legis] spes iustificationis, sed in sola fide, Rom. 3:28: Arbitramur justificari hominem per fidem, sine operibus legis” (Therefore the hope of justification is not found in them [the moral and ceremonial requirements of the law], but in faith alone, Rom 3:28: We consider a human being to be justified by faith, without the works of the law). Cf. In ep. ad Romanos 4.1 (Parma ed., 13.42a): “reputabitur fides eius, scilicet sola sine operibus exterioribus, ad iustitiam”; In ep. ad Galatas 2.4 (Parma ed., 13.397b): “solum ex fide Christi” [Opera 20.437, b41]).

See further:

Theodore of Mopsuestia, In ep. ad Galatas (ed. H. B. Swete), 1.31.15.

Marius Victorinus (ep. Pauli ad Galatas (ed. A. Locher), ad 2.15-16: “Ipsa enim fides sola iustificationem dat-et sanctificationem” (For faith itself alone gives justification and sanctification); In ep. Pauli Ephesios (ed. A. Locher), ad 2.15: “Sed sola fides in Christum nobis salus est” (But only faith in Christ is salvation for us).

Augustine, De fide et operibus, 22.40 (CSEL 41.84-85): “licet recte dici possit ad solam fidem pertinere dei mandata, si non mortua, sed viva illa intellegatur fides, quae per dilectionem operatur” (Although it can be said that God’s commandments pertain to faith alone, if it is not dead [faith], but rather understood as that live faith, which works through love”). Migne Latin Text: Venire quippe debet etiam illud in mentem, quod scriptum est, In hoc cognoscimus eum, si mandata ejus servemus. Qui dicit, Quia cognovi eum, et mandata ejus non servat, mendax est, et in hoc veritas non est (I Joan. II, 3, 4). Et ne quisquam existimet mandata ejus ad solam fidem pertinere: quanquam dicere hoc nullus est ausus, praesertim quia mandata dixit, quae ne multitudine cogitationem spargerent [Note: [Col. 0223] Sic Mss. Editi vero, cogitationes parerent.], In illis duobus tota Lex pendet et Prophetae (Matth. XXII, 40): licet recte dici possit ad solam fidem pertinere Dei mandata, si non mortua, sed viva illa intelligatur fides, quae per dilectionem operatur; tamen postea Joannes ipse aperuit quid diceret, cum ait: Hoc est mandatum ejus, ut credamus nomini Filii ejus Jesu Christi, et diligamns invicem (I Joan. III, 23) See De fide et operibus, Cap. XXII, §40, PL 40:223.

REFERENCE:

(Disputatio de controversiis: De justificatione 1.25 [Naples: G. Giuliano, 1856], 4.501-3):