What would you say?
What, I wonder would your assessment be? The most important/critical doctrine/teaching in the Bible? The letters of Paul to the Galatian and Roman Christians answer this question clearly: Justification by Faith in Christ Alone - or if you prefer, The Gospel.
Not just a "Reformation conviction"
This is not just a protestant answer flowing from our reformation heritage: it is the clear teaching Romans and Galatians, especially the latter. In the hot letter of Galatians, dispatched urgently to prevent churches in a whole region from deserting the Gospel, Paul shows the dramatic effects of the Gospel and the tragic effects of departure from it. Three of these effects, explained in chapters 3 and 4 are below...
1. The Gospel Unites the Church (any move from the Gospel divides it)
Since the only door into the Kingdom of God is faith in Christ, the Gospel has an amazing ability to unite people from every conceivable background. Jew, Gentile, men, women, upper class and lower class, or what other distinction you can come up with, all have to walk through the same door: and that makes them one in Christ. This is wonderful and completely unique. The moment a church adds to the Gospel anything (gifts of the Spirit, heavy doctrine, whatever), the church is divided into haves and have nots.
2. The Gospel causes us to think of God as our Father (a move back to law makes us think of him as a master)
During the era Moses-Messiah, God's people, though sons were in effect slaves, for as young sons they were under the care of a disciplinarian (the law). A son who is under such a disciplinarian (a pedagogos) is no different from a slave in practice. When Jesus came God's people grew up and received the Spirit of sonship. We are absolutely secure in our relationship with our Father in heaven. We do not go to bed thinking "I've blow it today, God doesn't love me." A son doesn't think like that, though a slave does. Going back to the law means going back to the era of law and the era of slavery and the era of master.
3. The Gospel allows the fruit of the Spirit to grow (a move back to law returns us to the era without the Spirit)
The church that moves back to law, moves back to the era without the Spirit, and therefore without the fruit of the Spirit. Such a church quenches the Spirit and is filled with back-biting and joylessness: everything opposite to the fruit of the Spirit.
There is no other doctrine which shapes the life of the Christia and the church more than justification by faith. And therefore we need Luthers to run through the church with new fire, not once every 15 centuries, but every week, so that the precious fruit of the Gospel will be maintained among the churches.