There is a doctrine which says that once you are saved you can never be lost, you cannot fall from grace. It is a popular doctrine, and is one of the five points of Calvinism. This belief relates to our 14th principle.
14th principle: The seal of God is conditional.
The doctrine that you cannot fall from graces means that God’s grace, and therefore his seal, is unconditional. The doctrine goes by various titles:
Now in a very real sense, the believer does have eternal security. Once the seal of God guarantees your inheritance (which it does immediately upon your baptism) then nobody can break that seal, or snatch you from God’s hand or rob you of your inheritance.
No temptation is able to overpower God’s own. They are secure in the hand of Jesus Christ (Romans 8:35-39, John 10:14,27-29,8:35-39, 1Corinthians 10:12-13).
However, that does raise the question of whether you, yourself, can ruin your own estate. What if you willfully sin and refuse to repent? Nobody can force you to, but what if you choose to do so? Is the seal still unbroken? Is your security still eternal? Or is your guarantee rendered void?
¶“4Here is something impossible. [I speak of] people who were once enlightened, who tasted of the heavenly gift. They became partakers of the Holy Spirit. 5They tasted the good word of God and the powers of the age to come. 6If people like that fall away, it is impossible to restore them to repentance again. They crucify God’s Son in themselves. They also disgrace him in public” (Hebrews 6:4-6).
Those who believe the fallen remain secure make an argument from the word "tasted" (Hebrews 6:4-6), saying the apostate was just a taster and not one who drinks fully of the Holy Spirit. The idea is that the taster is a pseudo-Christian, a pretender or dabbler rather than a true saint, and so was not sealed, and is therefore bound to fall.
This argument won't do, because Jesus himself was said to "taste death for every man" (Hebrews 2:9) yet we know he drank fully of that cup. This shows that when the Hebrew writer uses the word "tasted" he does not imply a mere sip or anything less than drinking fully.
¶“4Here is something impossible. [I speak of] people who were once enlightened, who tasted of the heavenly gift. They became partakers of the Holy Spirit. 5They tasted the good word of God and the powers of the age to come. 6If people like that fall away, it is impossible to restore them to repentance again. They crucify God’s Son in themselves. They also disgrace him in public” (Hebrews 6:4-6).
¶“7When the earth soaks up the frequent rain that falls on it, it produces useful plants for those who cultivate it. This ground receives God’s blessing. 8But ground that produces thorns and thistles is almost useless and cursed. It ends up being burned off” (Hebrews 6:7-8).
¶“9Although I speak like this to you whom I love, in your case I am convinced of better things —things that accompany salvation. 10God is not unfair. He won't forget your work and love for his name, your past and continuing service to his holy people. 11And we desire that each one of you show that same enthusiasm with the full assurance of hope all the way to the end. 12We don't want you to become dull. Rather, follow the example of those who, through faith and patience, inherit the promises” (Hebrews 6:9-12).
¶“19So my brethren, be bold to enter the most holy place with the blood of Jesus. 20[By that blood] Christ dedicated a new Way. This is a living way that enters through the veil. [I speak of 'the veil' figuratively] meaning the flesh of Jesus” (Hebrews 10:19-20).
¶“21Since we have a high priest over the house of God, 22let us draw near with a true heart in full assurance of faith. Let us have our hearts sprinkled from an evil conscience, and our bodies washed with pure water” (Hebrews 10:21-22).
¶“23Let us cling to the hope we confess. Let us not waver, for we can fully trust him who made the promises [on which we rest our hope]. 24Let us be mindful to stir one another to love and good deeds. 25Let us not abandon our gathering together like some do as a habit. Rather, let us encourage one another, especially as we see the day coming near.” (Hebrews 10:23-25).
¶“26Since we have received knowledge of the truth, we must not willingly continue to sin. If we do, no sacrifice for sins will be left for us. 27We can only be terrified, expecting the fire and fury of a judgment that will consume those who rebel. 28[We read in] the law of Moses that those who rebel against it die without mercy or appeal on [the testimony of] two or three witnesses. 29Don't you think that a far worse punishment is merited by one who tramples on the Son of God? One who has been made holy by the blood of Christ’s covenant, [should not] treat that blood as unholy. That would insult the Spirit of grace” (Hebrews 10:26-31).
¶“32Remember the early days after you first received the light. At that time you endured a great and painful struggle. 33You were in the public eye as victims of ridicule and persecution. Also when others had the same experience, you became their partners. 34You had compassion for those imprisoned. You accepted cheerfully the plundering of your goods, knowing that you have a better and abiding possession [reserved] for you in heaven” (Hebrews 10:32-34).
¶“35So don't throw away that confidence now. Its reward is great. 36You just need to keep enduring. After you have done the will of God, you will receive what he promised. 37'For in yet a little while, he who is coming will come, and he will not linger. 38Meanwhile my righteous ones shall live by faith, and if people shrink back, my soul will not be pleased with them.' 39But we won't shrink back with them to ruin. We will keep believing till our souls are saved” (Hebrews 10:35-39).
Look at Hebrews 6:4-12 and Hebrews 10:19-39 above, and note the state of those being discussed, that surely they were sealed and saved, yet they could fall. Here is what is said about them:
With all that, they could still sin willfully and come under punishment with the wicked (Hebrews 6:26).
Observe especially above that the apostate is defined as one "who has counted as an unholy thing the blood of the covenant by which he was sanctified." (Hebrews 10:29). Hebrews 10:14 has already taught us that those who are sanctified, are "perfected forever".
You cannot be more fully saved and sealed than that. Nevertheless, one who has been sanctified can repudiate that estate, and become a reprobate for whom "there remains no more a sacrifice for sins" (Hebrews 6:14,26).
The seal of God is forever, and the security of the believer is eternal, but only on the condition that one does not draw back to perdition.