I disagree with this idea of once save forever saved. In (1 Cor.9:27) The Apostle Paul wrote under the inspiration of the Holy Spirit, “But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.” How do we explain the word castaway? Paul knew that he was saved, if you are correct, how could Paul be a castaway?
Paul also wrote that when we first believed, we received the seal of the Holy Spirit as a deposit guaranteeing our inheritance (Ephes. 1:13-14), and that God has taken ownership of us and it’s He who makes us stand, (2 Cor. 1:21-22). Since, as you said, he was writing under the inspiration of the Holy Spirit then how could he contradict himself like this?
The Greek word translated castaway means not standing the test, or being disqualified. The only test for salvation is belief (John 3:16, John 6:28-29. John 6:40) and the only way to be disqualified is to refuse to believe (John 3:18, 2 Thes. 2:9-10).
In the context of 1 Cor. 9:25-27 Paul compared a believer’s life to an athlete’s attempts to achieve victory in a race. Both involve bringing one’s body into subjection. This is not a precondition for salvation, but a way to achieve victory over the flesh after we’ve been saved. Therefore this verse is not about salvation, but about living a victorious Christian life.
In 1 Cor. 3:15 Paul wrote that even if all our human works are burned up in the fire we’ll still be saved. In other words, even if all our attempts to put our body under subjection result in defeat rather than victory our salvation will not be affected.