The starting point must be the comparison that Paul draws between God’s attitude toward unbelieving Israel and his audience in Rome. My previous suggestion that a believer can indeed fall to their peril is based on this comparison in which Paul tells the Roman Christians that Israel was broken off for unbelief; thus, his warning to the Romans, “if God did not spare the original branches, perhaps he will not spare you” (11:21). For the apostle, believing Gentile Christians are liable to the same fate as Israel, namely God might cut them off. The comparison between the Roman church and Israel is developed through a contrast between the faith of the Romans and the unbelief of Israel, “They (Israel) were broken off because of their unbelief (Gk. apistia), but you stand only through faith (Gk. pistis)” (11:20). The explicit contrast of Paul’s Greek is somewhat muted in the English translation of “unbelief” vs. “faith” simply because English doesn’t have a negative word using the root “faith”. The Greek apistia vs. pistis is much stronger, and a more literal translation would say that Israel was “broken off because of their afaith (or unfaith?), but you stand only through faith.” This is enough to highlight the fact that, for Paul, if a person can move from God’s favor into condemnation, it is conditioned on whether or not that one continues in faith in Christ. The logic is quite clear. If it is through faith that we are united to Christ and brought into a state of reconciliation with God, then our unbelief would mean the breaking of our union with Christ, which would also mean that we no longer partake in the blessings of our former union.
Paul’s understanding of the contrast between faith and unbelief becomes increasingly clear when we consider Romans 4:20. Speaking of Abraham, Paul writes, “No unbelief (Gk. apistia) made him doubt the promise of God, but he was empowered by faith (Gk. pistis) giving glory to God.” Note the again the strong contrast between apistia and pistis. Abraham’s righteous standing before God is conditioned on his belief in the promise of God (4:21-22), and the opposite of this by-faith-righteousness is unbelieving condemnation. In Romans, Abraham is the prototypical Gentile believer, because he believed and was justified prior to his circumcision. So, faith in the crucified and risen Christ incorporates even a Gentile believer into Abraham’s family, which is defined around the Messiah. In contrast, unbelief cuts a person off from this family. This is precisely what Paul says happened with ethnic Israel, and in his thinking it is a danger to believers in Rome. Thus, his exhortation to continue in the kindness of God, which is conditioned on perseverance in faith, in order to avoid being cut off (11:22).
Returning to the initial question regarding how one falls away, we can say that the condition for being cut off from the people of God is unbelief or a cessation of faith in Christ. He does not here raise the issue of evil works as a means for falling away, though he would certainly assert that evil works are the product of unbelief. This makes sense in light Paul’s larger soteriology. If a person is justified by faith, then falling into unbelief would necessitate falling out of justified reconciliation with God. One might say that this is all hypothetical for Paul, and that a true believer will never fall into unbelief. The problem with that suggestion is that Paul doesn’t seem to be dealing in hypotheticals. His argument is based on the very concrete and historical example of God’s action to cut off unbelieving Israel. Cutting off, he insists, is the grievous consequence of unbelief. For Paul, it appears to be a real possibility that a justified true believer could fall into unbelief and be cut off from the people of God.