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. The doctrine goes by various titles:
The Impossibility of Apostacy
Infallible Grace
The Eternal Security of the Believer
The Perseverance of the Saints
Once Saved Always Saved
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 (Rom 8:35-39 Jhn 10:14, 27-29, 1Co 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?
The Argument on “Tasted of Grace”
Those who believe the fallen remain secure make an argument from the word "tasted"(Hebrews 6:4-11), 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"(Heb 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.
Examining the Argument
Look at Hebrews 6:4-11 and Hebrews 10:19-39, 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:
Were once enlightened (Heb 6:4)
Had tasted the heavenly gift (Heb 6:4)
Were made partakers of the Holy Spirit (Heb 6:4)
Had tasted the powers of heaven (Heb 6:5)
Had entered through the veil by the blood (Heb 10:19)
Had a great High Priest (Heb 10:21)
Hearts were sprinkled clean (Heb 10:22)
Had a confession of hope (Heb 10:23)
Had been sanctified by Christ's blood (Heb 10:29)
Had an enduring inheritance (Heb 10:34)
With all that, they could still sin willfully and be under punishment with the wicked (Heb 6:26).
The Crux of the Argument
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." (Heb 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" (Heb 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.
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."