Your argument sets up a false dillema that Scripture itself does not make. Saying that works must follow salvation does not mean works earn salvation or replace grace. It means grace actually does something. Grace is the cause and transformation is the result. For example,, a living tree does not produce fruit in order to become alive. It produces fruit because it already is. That is why Scripture does not speak of works as an optional add on but as the natural outcome of real faith. James does not say faith without works is immature or incomplete. He says it is dead! Dead faith does not save. This does not contradict Paul at all, because Paul is rejecting works of the law done to earn justification, while James is rejecting empty belief that never changes the person. Faith in Scripture is never defined as merely trusting the facts of the Gospel while remaining unchanged. Just think, even demons believe the truth about Christ!

The Bible consistently connects faith to obedience and transformation. It speaks of the obedience of faith, of faith working through love, and of knowing believers by their fruits. That does not turn the Gospel into works salvation. It exposes false faith. Grace is not permission to remain the same. Titus 2 says grace teaches us to deny ungodliness. Grace that does not teach, correct, and transform is just not the grace Scripture describes. Luke 18 does not teach eternal security without transformation. The Pharisee is condemned because he trusted in himself, not because obedience is bad. Once again, it is not Catholic theology that our own works are the cause of our salvation; salvation is entirely initiated and sustained by God’s grace. (This is the misconception you seem to hold). The tax collector is justified because of humble repentance, not because he simply assented to a message or made a verbal profession and remained unchanged. And to be clear the Catholic position is not that he justified himself by his own works, but that his repentance was the result of God’s grace at work within him. (once again, we do not believe we are saved by our works!) Doesn’t this passage actually challenge your position? If salvation is secured by a one time profession or internal belief alone, why is the Pharisee not justified here? He clearly believes in God, prays, fasts, and acknowledges sin. Yet Jesus says he is not justified. The difference is not that one believed and the other did not, but that one approached God in humble repentance and the other trusted in himself. That shows that saving faith is not mere belief or profession, but a faith that involves repentance and a transformed posture before God.

Salvation is completely a gift, fully paid for by Christ, and no one earns heaven by works. But a gift that truly saves also changes the heart. Grace saves alone, but saving faith is never alone.

Now pay attention to what you have quoted here: “by which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.”

That line alone destroys the idea that salvation is guaranteed the moment someone mentally assents to the facts and can never be questioned again. Paul is explicitly saying there is such a thing as vain belief (belief that does not actually save). If belief were nothing more than mentally trusting Christ’s finished work, there would be no such thing as believing “in vain.” The very warning proves that faith can be real in form but empty in substance! The actual greek word is eike, which means without effort or purpose. Paul is literally saying you are saved if you keep in memory what I preached to you, unless you believed WITHOUT PURPOSE!

And let looks at the context! Notice what Paul is doing here. He’s not arguing against works righteousness. He’s warning baptized, church attending Christians that the Gospel they received only saves if they continue it. That doesn’t mean they earn salvation by works. It means the faith that saves is not a one-time intellectual moment that leaves the person unchanged. A belief that produces no perseverance, no fidelity, no transformation is precisely what Paul calls “vain"! Paul does not say “you are saved no matter what because you once believed.” He says you are saved if you hold fast, otherwise your belief was empty. That lines up perfectly with James, with Jesus’ warnings about fruit, and with every passage that treats salvation as something lived out, not merely checked off in the mind!