This argument usually boils down to one side saying someone can lose their salvation if they return to and continue in willful sin and the other side saying that the person was never truly saved in the first place. Personally I am comfortable with either position. The third position is when the eternal security position degenerates into a "greasy grace" or "grace as a license to sin" position which states that someone can live in continual willful sin and still be saved because at one time they were saved and can not lose their salvation no matter what they do. This is the position that I have a problem with and believe is un-scriptural and gives people a false sense of security.
Here are a few scriptures for your consideration that talk about enduring to the end and about people who turn back to willful sin.
1.\tMatthew 10:22 - All men will hate you because of me, but he who stands firm to the end will be saved.
2.\tMatthew 24:9-13 - "Then you will be handed over to be persecuted and put to death, and you will be hated by all nations because of me. At that time many will turn away from the faith and will betray and hate each other, and many false prophets will appear and deceive many people. Because of the increase of wickedness, the love of most will grow cold, but he who stands firm to the end will be saved."
3.\tRomans 11:20-22 - But they were broken off because of unbelief, and you stand by faith. Do not be arrogant, but be afraid. For if God did not spare the natural branches, he will not spare you either. Consider therefore the kindness and sternness of God: sternness to those who fell, but kindness to you, provided that you continue in his kindness. Otherwise, you also will be cut off.
4.\t1 Corinthians 9:27 - No, I beat my body and make it my slave so that after I have preached to others, I myself will not be disqualified for the prize.
5.\t1 Timothy 4:1 - The Spirit clearly says that in later times some will abandon the faith and follow deceiving spirits and things taught by demons.
6.\tHebrews 3:12-14 - See to it, brothers, that none of you has a sinful, unbelieving heart that turns away from the living God. But encourage one another daily, as long as it is called "Today," so that none of you may be hardened by sin's deceitfulness. We have come to share in Christ if we hold firmly till the end the confidence we had at first.
7.\t2 Peter 2:20-22 - If they have escaped the corruption of the world by knowing our Lord and Savior 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.
8.\t2 Peter 3:17 - Therefore, dear friends, since you already know this, be on your guard so that you may not be carried away by the error of lawless men and fall from your secure position.
9.\tRevelation 3:5 - He who overcomes will, like them, be dressed in white. I will never blot out his name from the book of life, but will acknowledge his name before my Father and his angels.
10. Jud 1:3-6 Dear friends, although I was very eager to write to you about the salvation we share, I felt I had to write and urge you to contend for the faith that was once for all entrusted to the saints. (4) For certain men whose condemnation was written about long ago have secretly slipped in among you. They are godless men, who change the grace of our God into a license for immorality and deny Jesus Christ our only Sovereign and Lord. (5) Though you already know all this, I want to remind you that the Lord delivered his people out of Egypt, but later destroyed those who did not believe. (6) And the angels who did not keep their positions of authority but abandoned their own home--these he has kept in darkness, bound with everlasting chains for judgment on the great Day.
11. Matthew 7:19-23 - "Every tree that does not bear good fruit is cut down and thrown into the fire. Thus, by their fruit you will recognize them. "Not everyone who says to me, 'Lord, Lord,' will enter the kingdom of heaven, but only he who does the will of my Father who is in heaven. Many will say to me on that day, 'Lord, Lord, did we not prophesy in your name, and in your name drive out demons and perform many miracles?' Then I will tell them plainly, 'I never knew you. Away from me, you evildoers!'”
12. Heb 10:26-31 If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, (27) but only a fearful expectation of judgment and of raging fire that will consume the enemies of God. (28) Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses. (29) How much more severely do you think a man deserves to be punished who has trampled the Son of God under foot, who has treated as an unholy thing the blood of the covenant that sanctified him, and who has insulted the Spirit of grace? (30) For we know him who said, "It is mine to avenge; I will repay," and again, "The Lord will judge his people." (31) It is a dreadful thing to fall into the hands of the living God.
God bless,
Patrick
www.jonahproject.org