"For it is by grace you have been saved, through faith—and this not from yourselves, it is the gift of God— not by works, so that no one can boast"
This argument behind “Once Saved, Always Saved” comes down to one basic question;
Are we saved through Jesus' work on the cross, or are we saved because of our own works?
Jesus tells us that he is the way, the truth, and the life. Then he tells us that no man can come to the father except through him. (John 14:6) This means that the only way we can possibly be saved is through faith in Jesus Christ. There is no other way.
If we believe that we can do something,... ANYTHING to lose our salvation, then we are putting our faith in ourselves. Keep the doctrines of the bible to keep salvation? How is that any different than someone saying that you MUST be baptized in order to gain eternal life? Yet we all agree that baptism is not necessary for salvation, (at least I think we all agree.) God promises us eternal life if we just believe in our hearts that Jesus Christ died for our sins and defeated death three days later when he rose from the dead. God ALWAYS keeps his promises.
"And you also were included in Christ when you heard the word of truth, the gospel of your salvation. Having believed, you were marked in him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance until the redemption of those who are God's possession—to the praise of his glory."
The King James Version uses the word Earnest in place of deposit. When I was 20 years old, I took a comprehensive Real Estate course. (I had planned on becoming a real estate agent at one point in my life.) In the world of real estate, an earnest means a down payment, gauranteeing that you will complete the transaction at a later date. If for any reason, you do not follow through with the transaction, the other party gets to keep your earnest money. This is the same concept used here. God has given us the Holy Spirit as a down payment towards our salvation. It is a contract between us and God that he will make good on the promise of our redemption. If God for any reason goes back on his promise, we would be entitled to keep the Holy Spirit. That's means that we would take the Holy Spirit to Hell with us. The Holy Spirit in Hell?
The bottom line is this: We are saved by God's grace, and not our own works. We are saved through faith in Jesus Christ. He is the only way to the father. None of us deserve to go to heaven and we could never do enough good deeds to get there. If God has saved us, than it is final. If one is putting his faith in anything other than the sanctifying work of Jesus Christ, (including faith in himself to live the Christian life,) then I fear for that man's eternal salvation. Trust in Jesus and only Jesus. He IS the way, the truth and the life.