Does a believer lose their unconditional eternal security if they do not believe in unconditional eternal security?
Personally, I wouldn't call someone a heretic simply for believing that its possible for salvation to be forfieted.
Even Jesus taught that the unforgiving servant lost his salvation (Matt. 18:23-35). And Jesus also taught that the servant who starts getting drunk and beating the other servants will lose his salvation (Matt 24:48-51).
The Bible talks about those who have fallen from grace, shipwrecked their faith, departed from the faith, etc. Certainly the Apostle Paul was not teaching heresy!
Here are some of the passages that the Lord used to teach me almost a decade ago that OSAS was heresy:
But he that shall endure unto the end, the same shall be saved (Matt. 24:13).
From that time many of his disciples went back, and walked no more with him (Jn. 6:66)
Then said Jesus to the Jews which believed on him, if ye continue in my words, then are ye my disciples indeed
If a man abide not in me, he is cast forth as a branch, and is withered, and men gather them, and cast them into the fire, and they are burned (Jn. 15:6)
Paul exhorted them all, that with purpose of heart they would cleave unto the Lord (Acts 11:23).
Paul persuaded them to continue in the grace of God (Acts 13:43).
Confirming the souls of the disciples, and exhorting them to continue in the faith, and that we must through much tribulation enter into the kingdom of God (Acts 14:22).
Paul said, And you that were sometimes alienated and enemies in your mind by wicked works, yet now hath he reconciled in the body of his flesh through death, to present you holy and unblameable and unreproveable in his sight: If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven
Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear: for if God spared not the natural branches, take heed lest he also spare not thee. Behold therefore the goodness and the severity of God: on them which feel, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off (Rom. 11:20-21).
But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway (1 Cor. 9:27).
For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live (Rom. 8:13).
Wherefore let him that thinketh he standeth take heed lest he fall (1 Cor. 10:12)
Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand; by which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain (1 Cor. 15:1-2)
We then, as workers together with God, beseech you also that ye receive not the grace of God in vain (2 Cor. 6:1)
For now we live, if ye stand fast in the Lord (1 Thes. 3:8)
Take heed unto thyself, and unto the doctrine; continue in them: for in doing this thou salt both save thyself and them that hear thee (1 Tim. 4:16)
Therefore we ought to give more earnest heed to the things which we have heard, lest at any time we should let them slip. For if the word spoken by angels was stedfast and every transgression and disobedience received a just recompense of reward; how shall we escape, if we neglect so great salvation (Heb. 2:1-3)
But Christ as a son over his own house; whose house are we, if we hold fast the confidence and the rejoicing of the hope firm unto the end (Heb. 3:6)
Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. But exhort one another daily, while it is called today; lest any of you be hardened through the deceitfulness of sin. For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end (Heb. 3:12-14)
Let us labour therefore to enter into the rest, lest any man fall after the same example of unbelief (Heb. 4:11)
Seeing then that we have a great high priest that is passed into the heavens, Jesus the Son of God, let us hold fast our profession (Heb. 4:14)
Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water. Let us hold fast the profession of our faith without wavering
And to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaketh better things than that of Abel. See that ye refuse not him that speaketh. For if they escaped not who refused him that spake on the earth, much more shall not we escape, if we turn away from him that speaketh from heaven (Heb. 12:24-25)
Jesus spoke of those who receive the word and for a while believe but in time of temptation fall away (Lk. 8:13).
For if we sin willfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, but a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries. He that despised Moses law died without mercy under two or three witnesses: Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace? For we know him that hath said, Vengeance belongeth unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people. It is a fearful thing to fall into the hands of the living God (Heb. 10:26-31).
Brethren, if any of you do err from the truth, and one convert him; let him know that he which converteth the sinner from the error of his way shall save a soul from death, and shall hide a multitude of sins (Jas. 5:19-20).
For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning. For it had been better for them not to have known the way of righteousness than, after they have known it, to turn from the holy commandment delivered unto them (2 Pet. 2:20-21).
Ye therefore beloved, seeing ye know these things before, beware lest ye also, being led away with the error of the wicked, fall from your own stedfastness (2 Pet. 3:17).
Let that therefore abide in you, which ye have heard from the beginning. If that which ye have heard from the beginning remain in you, ye also shall continue in the Son, and in the Father (1 Jn. 2:24).
Whosoever transgresseth, and abideth not in the doctrine of Christ, hath not God. He that abideth in the doctrine of Christ, he hath both the Father and the Son (2 Jn. 1:9).
He that keepeth the commandment keepeth his own soul; but he that despiseth his ways shall die (Prov. 19:16).
Paul said that if a believer violates their conscience, even by simply eating meat offered to idols when they believe it is wrong, that they commit sin and are therefore damned (Rom. 14:23).
Paul said that those believers who do what they believe to be wrong are in danger of being among those who perish (1 Cor. 8:11).
And the Lord said unto Moses, whosoever hath sinned against me, him will I blot out of my book (Exo. 32:33).
He that overcometh, the same shall be clothed in white raiment; and I will not blot out his name out of the book of life, but I will confess his name before my Father, and before his angels (Rev. 3:5).
The Bible says that the wrath of God commeth upon the children of disobedience. Be not ye therefore partakers with them (Eph. 5:6-7).
The Bible says, For the wrath of God is revealed from heaven against all ungoldiness and unrighteousness of men (Rom. 1:18), and the Bible says, indignation and wrath, tribulation and anguish, upon every soul of man that doeth evil (Rom. 2:8-9).
Paul even warned believers, But after thy hardness and impenitent heart treasurest up unto thyself wrath (Rom. 2:5).
Consider the warnings the Bible gives against whosoever. The Bible says whosoever will be in danger of hell if they break the commandments and teach others to do so (Matt. 5:19); if they are angry with a brother without cause or calls them a fool (Matt. 5:22); if they commit adultery by looking with lust (Matt. 5:28); if they publicly deny Jesus Christ (Matt. 10:33); if they blaspheme the Holy Spirit (Matt. 12:32); if they seek to save their life (Matt. 16:25); if they cause little children to stumble (Mk. 9:42); if they commit sin (Jn. 8:34-35); if they judge others in hypocrisy (Rom. 2:1); if they resist lawful government (Rom. 13:2); if they try to earn their salvation by their works (Gal. 5:4); if they are a friend of the world (Jas. 4:4); if they hate their brother (1 Jn. 3:15); if they do not abide in Christ and in His doctrine (2 Jn. 1:9); and if they love or make lies (Rev. 22:15). These passages do not warn unbelievers only, but specifically warns whosoever which means everyone.
Paul said that we are chastened of the Lord, that we should not be condemned with the world (1 Cor. 11:32).
But if that evil servant shall say in his heart, my lord delayeth his coming; and shall begin to smite his fellow servants, and to eat and drink with the drunken; the Lord of that servant shall come in a day when he looketh not for him, and in an hour that is not aware of, and shall cut him asunder, and appoint him his portion with the hypocrites: there shall be weeping and gnashing of teeth (Matt. 24:49-51).
And grieve not the Holy Spirit of God, whereby ye are sealed unto the day of redemption (Eph. 2:30).
I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel (Gal. 1:6).
Being blotted out of the book of life is a real possibility (Exo. 32:32-33; Deut. 29:20; Ps. 62:20; Rev. 3:5).
The Lord is with you, while ye be with him
if ye forsake him, he will forsake you (2 Chron. 15:2).
Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor homosexuals
. Shall inherit the kingdom of God (1 Cor. 6:9-10).
Love not the world, neither the things that are in the world. If any man love the world, the love of the Father is not in him (1 Jn. 2:15).
The Bible repeatedly speaks of those who have fallen away from the faith (Matt. 18:21-34; Mk. 4:17; Lk. 8:13; Jn. 6:66; Acts 1:25 w. Matt. 19:28; 2 Thes. 2:3; 1 Tim. 1:5-6; 1:19-20; 4:1; 5:15; 2 Tim. 3:8; 4:10; Heb. 3:12-15; 4:1-11; 6:6; 10:29; 2 Pet. 2:20-22; Jud. 1:5).
Holding faith, and a good conscience; which some having put away concerning faith have made shipwreck
(1 Tim. 1:19).
Paul said, To them who by patient continuance in well doing seek for glory and honour and immortality, eternal life: but unto them that are contentious, and who do not obey the truth but obey unrighteousness, indignation and wrath, tribulation and anguish, upon every soul of man that doeth evil, of the Jew first, and also of the Gentile; But glory, honour, and peace, to every man that worketh good, to the Jew first, and also to the Gentile (Rom. 2:7-10).
For if I build again the things which I destroyed, I make myself a transgressor (Gal. 2:18).