Luther stresses the "paradox" (contraria) by stating that the verdict of law and gospel are absolutely contradictory (contradictoria). He says, "These two things are diametrically opposed [ipsa ex diametro pugnant]: that a Christian is righteous and beloved by God, and yet he is a sinner at the same time. For God cannot deny His own nature. That is, He cannot avoid hating sin and sinners; and He does so by necessity, for otherwise He would be unjust and would love sin. Then how can these two contradictory things both be true at the same time, that I am a sinner and deserve divine wrath and hatred and that the Father loves me? Here nothing can intervene except Christ the Mediator." This last simple sentence explains the paradox. It explains the whole Christian religion. It explains the Scriptures. It is the secret to all exegesis of Scripture and all theologizing. It is the only comfort that a poor sinner has in life and in death. It is "Christ alone." So we have in the principle of solus Christus not only a hermeneutical rule, not only the basis for all comfort, not only the basis for our union with God and for reconciliation and salvation, but the principle of all human knowledge and understanding.
Preus, Robert D., "Luther: Word, Doctrine, and Confession." Concordia Theological Quarterly 60.3 (1996): 175-228.