"The cross of Christ is the only instruction in the Word of God there is, the purest theology."

"Crux Christi unica est eruditio verborum dei, theologia sincerissima" (AWA 2, below).
"Crux Christi unica est eruditio verborum dei, Theologia syncerissma" (WA 5, below).
"Crux Christi unica est eruditio verborum Dei, theologia sincerissima" (E ex XIV, 330).

The cross of Christ is the only (acquired) knowledge of the words of God there is, the purest theology.

     Martin Luther, Operationes in Psalmos (1519-1521), at Ps 6:11 (Operationes in Psalmos, 1519-1521, Teil II:  Psalm 1 bis 10 (Vulgata), ed. Gerhard Hammer and Manfred Biersack, Archiv zur Weimarer Ausgabe der Werke Martin Luthers, Texte und Untersuchungen, Bd 2 (Köln and Wien:  Böhlau Verlag, 1981), 389, ll. 15-16; WA 5, 217, ll. 2-3; E ex XIV, 330), as translated by Robert Kolb (“Luther on the theology of the cross,” chap. 1 in The pastoral Luther: essays on Martin Luther’s practical theology, ed. Timothy J. Wengert (Grand Rapids, MI: William B. Eerdmans Publishing Company, 2009), 34 (33-56); Martin Luther:  confessor of the faith (Oxford:  Oxford University Press, 2009), 58).  Luther's commentary on the first twenty-two Psalms, based on Dr. Henry Cole's translation from the original Latin.  Revised, enlarged, parts retranslated and edited in complete form by John Nicholas Lenker, D.D., Standard edition of Luther's Works (Sunbury, PA:  Lutherans in All Lands Co., 1903), 361:

“The cross of Christ is the only way of instruction in the Word of God, and the only true theology."

     I was put onto that sentence by Dr. Kerry Dearborn.  Cf. "The CROSS alone is our Theology":

"CRUX sola est nostra Theologia" (WA 5, 176). 

"Crux sola est nostra theologia" (E ex XIV, 261).

Operationes in Psalmos, at Ps 5:12.

     AWA 2 cites also, in this connection, the Duo sermones de passio Christi (1518, WA 1, 341, ll. 37-40, scheduled for translation in vol. 3 (?) of the New series of the American edition of Luther's Works):

Nam incarnatio et passio Christi, licet ad omnem affectum et intellectus eruditionem nobis commendentur inspicienda, maxime tamen ad caritatem Dei inspiciendam cognoscendamque erga nos per Scripturam nobis exhibentur contemplanda.

AWA 2 references also, on the "theologia sincerissima", "L-Vorrede, o 7, Anm 61."