The phrase "once saved, always saved" is not found in the Bible. Our salvation is past, present, and future. That implies a continuous relationship and an unfolding process rather than a fixed state. The person who has given himself or herself to Jesus Christ and has truly repented from sin can find repeated assurances of the eternal covenant given to His people by a God who cannot lie. The Bible tells us that the gifts and calling of God are without repentance (see Romans 11:29). The work of Jesus Christ is eternal. It will never be revoked. Apart from the Bible, the greatest assurance any Christian has of eternal security is the day-by-day witness of the Holy Spirit in his life that he is a child of God and the fact that day-by-day he is becoming more like Jesus.
The Bible teaches security in the Holy Spirit and in the things of God. Jesus said, "My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of My Father's hand" (John 10:29). So we are secure in the hand of God, and we are to rest secure in the Lord. God has put His Spirit within us; He has given His Word to us. Jesus Christ is at the right hand of God, constantly making intercession on our behalf; (see Hebrews 7:25) and God has sent angels to watch over us (see Hebrews 1:14). In other words, we are surrounded by tremendous helps. We should constantly remind ourselves that we are secure, that God loves us, and that we belong to Him.
The reason we are saved is not because we are holy or do good works, but because of the blood of Jesus. Salvation comes through faith, and it stays because of faith. So we should have a feeling of security. But, at the same time, there should be a healthy fear of falling. We should not presume on God and say, "I am saved, so I can do anything I want to." Paul condemned people who were teaching that a person could sin boldly so that God's grace could abound (see Romans 6:1-2). If we truly are dead to sin, then we are not going to live in sin any longer, but we will aim to serve God.
We should walk with reverence, because God is a consuming fire, a very awesome Being (see Hebrews 12:29). I would not counsel anybody to presume on God because of a doctrine of eternal security. Anybody who says, "I believe that once I am saved, I am always saved, so I am going to go out and commit adultery and steal and lie and cheat and rob," is being very foolish. If somebody really loves God, those things would not be present in his life.
I have walked with God for nearly thirty years, and for the last twenty-five years or so, I have never even thought about the possibility of losing my salvation. It just is not a reality for me at all. The reason is that I live for the Lord. The thought of doing otherwise just does not occur to me. Do I commit acts of sin? Yes. Do I fail God? Yes. Are there things I should do that I do not do? Yes. I need the cleansing forgiveness of the Lord constantly. My concern is that I realize God's purpose for my life and that I do not fall short of what He intends for me. But as far as losing my salvation, it just does not come up for me, nor should it for any sincere Christian.