There is no "once saved always saved" in the bible.
Salvation is a life long process, not a one time thing of saying you believe in Jesus.
The apostle John wrote us a wonderful promise from God: John 3:16-17 "For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life."
Does this mean that once we accept Jesus in our heart by faith, and thus are saved, that we can do anything even immoral things, and we still will be saved? Let's see what the Scriptures says about this.
1 Tim 4:1: "The Spirit clearly says that in later times some will abandon the faith and follow deceiving spirits and things taught by demons."
This text shows that Christians can loose their salvation if they follow deceiving spirits.
2 Peter 2:20-22: "If they have escaped the corruption of the world by knowing our Lord and Saviour Jesus Christ and are again entangled in it and overcome, they are worse off at the end than they were at the beginning. It would have been better for them not to have known the way of righteousness, than to have known it and then to turn their backs on the sacred command that was passed on to them. Of them the proverbs are true: 'A dog returns to its vomit," and, "A sow that is washed goes back to her wallowing in the mud.'"
The apostle Peter indicates clearly here that it is worst for someone who has accepted Jesus in his life to return to his old ways than "not to have known the way of righteousness". Why? The book of Hebrews explains this for us.
Heb 6:4-6: "It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age, if they fall away, to be brought back to repentance, because to their loss they are crucifying the Son of God all over again and subjecting him to public disgrace."
This text describes Christians who not only accepted Jesus by faith into their life, but also personally tasted God's Holy Spirit. What happens to them if they "fall away'? This text is clear. It is impossible for them to be brought back to repentance, and thus obtain eternal salvation, because they did sin against the Holy Spirit. They have lost it!
Heb 10:26-28: "If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, but only a fearful expectation of judgment and of raging fire that will consume the enemies of God."
Here we see clearly that once we have accepted Jesus in our life, we just can't go on living as if He did not exist, and "deliberately keep on sinning". What would be the result if you did? Would you still obtain eternal salvation? The Bible says that these will have "a fearful expectation of judgment". There again, they have lost their salvation.
The apostle Paul confirms this in 1 Cor 15:1-2: "Now, brothers, I want to remind you of the gospel I preached to you, which you received and on which you have taken your stand. By this gospel you are saved, if you hold firmly to the word I preached to you. Otherwise, you have believed in vain."
In other words, if someone does not hold on firm to God's word, he/she has "believed in vain".
An example that one can loose his/her salvation is King Saul. He was chosen by God to be the first king of Israel. He was filled with God's Spirit at first (1 Samuel 10 6), but because of his pride and evil actions, never repenting from what he did wrong, eventually God's Spirit departed from him 1 Sam 16:14 "Now the Spirit of the LORD had departed from Saul…" NIV King Saul had been saved, but he lost his salvation due to his unrepentant heart. How sad!
These texts clearly contradict the theory of "Once saved, always saved". This theory is simply not Biblical.
"As the Bible says, I am already saved (Rom. 8:24, Eph. 2:5–8), but I’m also being saved (1 Cor. 1:8, 2 Cor. 2:15, Phil. 2:12), and I have the hope that I will be saved (Rom. 5:9–10, 1 Cor. 3:12–15). Like the apostle Paul I am working out my salvation in fear and trembling (Phil. 2:12), with hopeful confidence in the promises of Christ (Rom. 5:2, 2 Tim. 2:11–13)."