We've already started to contemplate what Luther called the Theology of the Cross (TC) which he explains as the center point of and fountain from which all other Christian doctrine flow. TC rests heaviest on the “first and chief article of the Christian Faith” – upon Justification (“to render as righteous”). The church and all other doctrines stand or fail on justification as the external, imputed forensic status of righteousness which the Christian embraces through faith alone.
I believe that to put Luther's TC doctrine into perspective; one also needs a sound understanding of Luther's loathing for the transgression of the Law and remorse over sin. (RC Sproul, Sr's lectures on the Holiness of God continues as my favorite resource on this point.) From that understanding, Luther teaches that the Old Covenant Law which was once our condemnation becomes instead a mirror which shows us our sin in stark terms (through the Law and Gospel).
As we behold our sin and realize who God is, our sin drives us back to the Gospel message of all that Christ has done for us, and only there, we find love and comfort in repentance and assurance in forgiveness based on our faith alone.
So to Luther, the Gospel message becomes any Scripture that depicts how God emptied himself, became nothing, only to suffer and die in humility on the Cross. In the Gospel, we realize that the Man of Sorrows has laid down His life for us that we might be ransomed from the wrath that justice demands and find eternal life.
Sanctification proceeds from justification, and through the daily process of renewal at the Cross, the Holy Spirit works in us spontaneously to produce good works.
In many ways, John Calvin further clarifies what Luther leaves unwritten, and he returns to a more patristic approach to justification and sanctification which Luther resisted. Calvin was a student of Law which at that time involved critique of the Classical Greco-Roman writings of philosophy, and he trained in the humanistic method of the rising French Reform movement. Many hoped that it could change the Roman Catholic Church gradually from within it. For many years, Protestant ideas were developed from within the church in France (until they were no longer tolerated).
Pursuant to that cause, Calvin develops the theme of a mystical, spiritual Union with Christ which seeks to treat sanctification as fully as Luther does justification so that both concepts stand each on their own merit. Calvin used it to echo the Apostle Paul's 216 mentions of the term “in Christ” or “in Him” within his epistles to liken salvation to God's adoption of the sinner into a rich inheritance of love through the agent of the Holy Spirit.
Calvin called this Union with Christ "the Sum of the Gospel." from which flows justification first, then sanctification – both inseparable concepts that are distinguishable. He called this the Double Benefit of grace. He borrows a patristic concept to describe justification as Luther does as a forensic declaration of God's adoption of the sinner, linking it with Christ's humanity.
This first Benefit forms the Christological aspects of life in Christ and focuses on the suffering of Christ and the Cross. The second benefit of Sanctification approaches the analogy of adoption from a Trinitarian view which unites the Believer through the Spirit with God as a loving father who bestows all the benefit of inheritance on His beloved child. Part of that adoption includes welcome into the life of the ecclesia in the covenant community and the brotherhood within the church. Calvin qualifies sanctification as an internal guiding through the Spirit into a life characterized by obedience – to "cultivate blamelessness and purity of life.”
