ANSWER: It is true that once you are saved you are always saved. The question, however, is WHEN is a person saved for eternity? The Biblical answer regarding this important question on whether or not a person can lose their salvation is significantly different from what Catholics and almost all the mainstream Protestant denominations in the world teach.
The commonly taught doctrine of 'once saved always saved' comes primarily from John Calvin and his teachings regarding the 'preservation of the saints.' This salvation doctrine teaches that once God calls a person they are assured entrance into heaven NO MATTER HOW they live their life after this calling.
The Biblical truth is that once a Christian has repented and turned his life around he is ON HIS WAY to ultimate salvation. He still has to live, however, a lifetime journey of trials, troubles, and overcoming sin before he can fully receive it! The acceptance of Jesus Christ and the receiving of God's Holy Spirit is the start, not the end, of the process of receiving life forever.
A converted Christian is REQUIRED to produce the fruits of overcoming for however long they live (Revelation 2:11). They must keep God's commandments, both in the letter and in the spirit (Matthew 19:17, James 2:14 - 17, Revelation 12:17, 14:12, 22:14, etc.). Before someone receives the gift of eternal life they must also endure and remain faithful to the END (Matthew 24:13).
Paul, in the book of Hebrews, not only states that believers can reject God but also what will occur after the choice is made! His inspired words make NO SENSE if we think that once a person 'accepts Jesus' they are GUARANTEED to live eternally no matter how they live the rest of their lives.
4. For it is IMPOSSIBLE for those who were once enlightened, and who personally obtained the heavenly gift, and became partakers of the Holy Spirit (they fulfill the Biblical definition of a true Christian), 5. And who have tasted the good Word of God (their mind has been opened to what God's word teaches), and the powers of the world to come (they understand what is the destiny of those who love God), 6. If they have fallen away (they choose to reject God), to renew them again unto repentance (they commit the unpardonable sin)
26. For if we willfully go on sinning AFTER receiving the knowledge of the truth (note that believers can choose to AGAIN lead a life of sin after conversion), there is NO LONGER any sacrifice for sins (no more forgiveness through the sacrifice of Christ), 27. But a terrifying expectation of inevitable judgment and of fierce fire (the second death) . . . (Hebrews 6:4 - 6, 10:26 - 27, HBFV throughout)
Paul understood that he needed to remain faithful to the very end of his life or else even HE would be rejected!
27. But I discipline my body and bring it into subjection, lest, after preaching to others, I myself might be REJECTED (1Corinthians 9:27).
It was ONLY near the very end of his life, when martyrdom was imminent, that Paul knew that salvation was awaiting him.
6. For I am now ready to be offered, and the time of my departure is at hand. 7. I have fought a good fight; I have finished the course; I have kept the faith. 8. From this time forward, a crown of righteousness is laid up for me, which the Lord, the righteous Judge, shall give me IN THAT DAY. . . (2Timothy 4:6 - 8)
Paul died in the faith and awaits in the grave, with all the saints, the receiving of the gift of life eternal given at the return of Jesus Christ to earth (1Corinthians 15:50 - 55).
Man is endowed by his Creator with the power to make choices. All humans possess free (though not perfect) will. A Christian always has the freedom to walk away from or lose salvation after having gained it. Believers can always choose to be 'lost' again at any time before he or she dies. Other passages to study regarding this topic include Matthew 24:13, 1Corinthians 15:2, Hebrews 2:3, 3:6, 12:4, 4:11, 10:35 and 12:25, James 5:19 - 20, 2Peter 2:20 - 22 and many others.
Salvation is a process that takes up most of a person's physical lifetime. By overcoming sin, Satan's influence in this world and themselves, believers become mature and spiritually perfect, as God is perfect. By working out their calling with 'fear and trembling' (Philippians 2:12), Christians can insure they do not lose the glorious gift of existing forever in love.