Some other verses that confound Protestants.
1 Cor 9:27 But I chastise my body, and bring it into subjection: lest perhaps, when I have preached to others, I myself should become a castaway.
Phil 2:12 Wherefore, my dearly beloved, (as you have always obeyed, not as in my presence only, but much more now in my absence,) with fear and trembling work out your salvation.
St.Paul was very far from the heresies of Luther and Calvin like "once saved, always saved" or even worse, "eternal security", "unconditional election" and the like novelties. When pursued to their logical conclusions, these doctrines are fatal to the practice of true Christian discipleship.
Justification is not something merely forensic and external as they vainly imagined, it is interior and transformational. Faith, says the Council of Trent, is the foundation and root of justification, but good works are necessary for salvation after baptism.
God not only actually blots out our transgressions and makes us a new creation, He also wants to destroy the power of sin and concupiscence over our lives, that we the branches may bear much fruit by the power of Him the Vine, which works are truly meritorious as well unto eternal life.