ANSWER: The Biblical answer regarding whether or not a person can lose their salvation is significantly different from what Catholics and almost all the mainstream Protestant denominations teach.
While it is true that once saved a person is always saved, the question is when does this sealed and unchangeable salvation state take place for a believer.
The commonly taught doctrine of 'once saved always saved' comes primarily from John Calvin and his teachings. He promoted the idea that a person cannot lose their salvation once God calls them. After being called they are assured entrance into heaven no matter how they live their life after this event!
The truth is that a person certainly can lose their salvation after they are called and receive God's Holy spirit!
A person who has accepted Jesus as their Savior has merely begun their training in righteousness. They must first live a righteous life and remain faithful to the end before they fully receive their reward. Along this journey there is the possibility they could harden themselves against God and forfeit or lose the neverending life they were promised.
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.). As stated previously, before someone receives the gift of eternal life they must also endure and remain faithful to the very end of their life (Matthew 24:13).
Paul not only states that true believers can reject God but also what will occur after their choice is made. He makes it abundantly clear that Christians can corrupt themselves to the point where it is impossible for them to repent. God is not bound by any oath to give them salvation no matter what they do and they lose out on eternity.
Those who reject the Lord while converted place themselves in a spiritual state that is far worse than before their conversion (2Peter 2:20 - 22). They not only lose all they had spiritually gained, they wait in fear for God's judgment befitted their error.
For it is IMPOSSIBLE for those who were once enlightened (their minds were open to understand the truth) . . . and became partakers of the Holy Spirit (they have God's spirit in them!), and who have tasted the good Word of God (they understood the rewards of holiness) . . . if they have fallen away (totally reject God), to renew them again unto repentance . . .
For if we willfully go on sinning after receiving the knowledge of the truth, there is no longer any sacrifice for sins (we lose our forgiveness), but a terrifying expectation of inevitable judgment and of fierce fire (the second death) . . . (Hebrews 6:4 - 6, 10:26 - 27, HBFV).
Those who have God's spirit but who willingly and willfully reject him have committed the unpardonable sin. Paul understood that he needed to remain faithful to the very end or else even he, too, would be rejected and lose out on salvation! He stated, "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 end of his life, when martyrdom was imminent, that Paul knew that salvation was awaiting him (2Timothy 4:6 - 8). He died in the faith and awaits, with all the saints who were obedient their entire lives, the receiving of the gift of life eternal (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. Other passages related to the topic of losing eternal life are 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 are trained to be spiritually mature.
By working out their calling with 'fear and trembling' (Philippians 2:12), to the end of their lives, Christians can insure they do not lose the glorious gifts and salvation they have been promised.