If all doctrine is one, if all the articles of doctrine are one and one is all, if doctrine is like a perfect golden ring, then Christ as the center is the whole essence of Christian doctrine just as he is the center and heart of the Scriptures. The solus Christus is not an abstraction but a reality embracing everything that Christ has done to save fallen mankind. The solus Christus embraces the entire work of God from creation to Christ’s return. It is the total opus ad extra of the Trinity. The solus Christus embraces not merely the work of Christ and the Father who sends Him, but also the work of the Spirit who sanctifies us. In fact, it is Christ who is our sanctification as well as our righteousness. The unity of doctrine is both christological and doctrinal, for the doctrine is Christ’s and Christ is the center of all the doctrine, perfecta doctrina. To Luther, then, the solus Christus dominates every article of faith, whether it is creation, redemption, the sacrament of the altar, baptism, worship, or whatever. It also dominates the third article. Christ is not only our righteousness, He is our holiness. Luther says, “The church is indeed holy, but it is a sinner at the same time.” Here simul justus et peccator becomes simul sanctus et peccator. Luther goes on in this way, “Therefore it believes in the forgiveness of sins and prays, ‘forgive us our debts’ (Matthew 6:12)… Therefore we are not said to be holy formally as a wall is said to be white because of its inherent whiteness. Our inherent holiness is not enough. But Christ is the perfect and total holiness of the church [perfecta et tota sanctitas ipsius]. When our inherent holiness is not enough, Christ is enough [satis est Christus].”
Robert D. Preus, “Luther: Word, Doctrine, Confession”, from Doctrine is Life: Essays on Scripture, pages 284-285.