John Calvin:

…justification by faith [is] the first and keenest subject of controversy…Wherever the knowledge of it is taken away, the glory of Christ is extinguished.

And later:

How comes it that we are “carried about with so many strange doctrines” (Hebrews 13:9)? Because the excellence of Christ is not perceived by us.

So of first importance is the gospel (1 Cor. 15:3) and the glory of Christ. That’s a message for today, too!