Apostle Paul gave an example in Hebrews 6 of a believer turning away from Christ, which is a lesson against the OSAS thinking...
Heb 6:1-11
6:1 Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God,
2 Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment.
3 And this will we do, if God permit.
This isn't about the basic principles of Christian doctrine, this subject he's getting ready to cover is about something that is unforgivable.
4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
5 And have tasted the good word of God, and the powers of the world to come,
6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put Him to an open shame.
For those who have experienced the heavenly calling and the gifts of The Holy Spirit of the world to come, they have no excuse if they should fall away from Christ, because for them to try to repent after doing that is like crucifying Christ all over again and putting Him to an open shame.
This is a very serious warning brethren against falling away, especially if you have a calling with spiritual gifts. An example of this type of falling away would be to renounce Christ, or worship another in His place (like the coming Antichrist).
Once one is shown that enlightenment, tasted of the heavenly gift, made partaker of The Holy Spirit, tasted of the good word of God, and the powers of the world to come, then there is no excuse for that one if they should fall away after that. It's about accountability and proof. To experience those things is God's direct proofs to the believer so that there is no longer any doubt. It is the level of sureness that those in Christ's future Millennial reign will see, and thus seal their accountability to either being in Christ, or being cast into the lake of fire.
7 For the earth which drinketh in the rain that cometh oft upon it, and bringeth forth herbs meet for them by whom it is dressed, receiveth blessing from God:
8 But that which beareth thorns and briers is rejected, and is nigh unto cursing; whose end is to be burned.
Paul gives an example in nature of how natural things compare with this, i.e., even thorns and briers that come forth and produce nothing good are cursed and to be burned.
9 But, beloved, we are persuaded better things of you, and things that accompany salvation, though we thus speak.
10 For God is not unrighteous to forget your work and labour of love, which ye have shewed toward His name, in that ye have ministered to the saints, and do minister.
11 And we desire that every one of you do shew the same diligence to the full assurance of hope unto the end:
KJV
Paul then admonishes the brethren not in that situation of falling away to remain dilligent unto the end, i.e. when Jesus comes. That is to say, they are capable of this unforgiveable sin, but he was persuaded of better things for them involving salvation.