CRUX sola est nostra theologia.

Luther, WA 5.176.32-33.

As Luther remarks, Anfechtung, 'in so far as it takes everything away from us, leaves us nothing but God: it cannot take God aweay from us, and actually brings him closer to us.' It is through undergoing the torment of the cross, death and hell that true theology and the knowledge of God come about. 'The Cross alone is our theology (CRUX sola est nostra theologia). It is only by experiencing the wrath of God in this manner that a man becomes a 'theologian of the cross.' It is precisely this consideration which underlies Luther's celebrated statement concerning the qualifications of a true theologian: 'living, or rather dying and being damned make a theologian, not understanding, reading or speculating' (vivendo immo moriendo et damnando fit theologus, non intelligendo, legendo aut speculando).

McGrath, Luther's Theology of the Cross, p. 152.