There is actually zero contradiction btwn salvation by grace thru faith and that of losing what faith obtained by rejection of it. That is not "saved by grace, kept by works theology" for you are not simply saved by grace, as if you woke up one morning as regenerate, nor by a faith which does not effect obedience to its Object, the Lord Jesus, nor is that lost every time we sin.
Instead the redeemed are those who come to God as sinners knowing their desperate need of salvation - not as souls saved by their works or religious heritage, but as destitute of any means or merit whereby they may find salvation - and with a humble and penitent heart (that at least implicitly wants a new life following Christ) believe on the crucified and risen Lord Jesus who alone can save them on His account, by His sinless shed blood and righteousness. (Romans 3:9 - 5:1
And who are thus baptized to follow the Lord Jesus with persevering faith (John 10:27-29; Acts 8:12; Ephesians 1:7; 2:8-10) - and who confess sin with effectual repentance when they are convicted by conscience of doing or being the opposite. (Psalms 32:5, 6; 51:3; Hosea 5:15; 1 John 1:9)
For effectual penitent, heart-purifying, regenerating faith, (Acts 10:43-47; 15:7-9) is that which is imputed for righteousness. (Romans 4:5) All those who came to God with this faith are those whom the Spirit of God thus spiritually baptized into the body of Christ, the church. (1 Cor. 12:13)
And those who die in that obedient faith will go to be forever with Him at death or His return (Phil 1:23; 2Cor. 5:8 [“we”]; Heb, 12:22,23; 1Cor. 15:51ff'; 1Thess. 4:17) In contrast to those who were never born of the Spirit or who terminally fall away. (Gal. 5:1-4; Heb. 3:12; 10:25-39) `
Thus even though we are not aware of many sins we commit, as well as character defects, yet we are still saved by such justifying faith, which effectually asks for forgiveness when they are convicted by conscience of sin. As David immediately did when fingered by Nathan (though he must have put his conscience in neutral concerning this for a while). (2 Samuel 12:1) Thus he could say "I have kept the ways of the Lord, and have not wickedly [râsha‛] departed from my God." (2 Samuel 22:22)
Christians are thus exhorted to "Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates?" (2 Corinthians 13:5)
For living contrary to faith is that of denying it (1 Tim. 5:8) refusal to repent when convicted of sin and die in that condition is to be lost, having forfeited what effectual penitent, heart-purifying, regenerating justifying faith obtain, against which Scripture clearly warns Christians, as Christians, doing.
And thus "we are judged, we are chastened of the Lord, that we should not be condemned with the world" (1 Corinthians 11:32) by dying impenitent.
For there is the sin unto death for believers, (1 John 5:16, 17; cf. the "great transgression" - Psalms 19:13) that of willfully impenitently - despite warnings and chastisement - departing from the faith once held, making Christ of no effect, to no profit, drawing back to perdition, thus forfeiting what faith appropriated, which is what believers, as believers, are warned against. (Gal. 5:1-5; Heb. 3:12; 10:25-39) For believers, there is the sin unto death, (1 John 5:16, 17) of willfully impenitently - despite chastisement - departing from the faith once held, making Christ of no effect, to no profit, drawing back to perdition, thus forfeiting what faith appropriated, which is what believers, as believers, are warned against. (Gal. 5:1-5; Heb. 3:12; 10:25-39)
I myself once held to OSAS, however we are called to follow the Lamb, and thus the Truth, wherever He leads, (Rv. 14:4) and that often can mean where we would rather not go, and thus at least in this I was compelled to change my conclusion in response to more illumination. Note that the usual recourse to assigned texts as below unbelievers is simply not tenable, and amounts to wresting the texts, which are written to believers to continue in the faith, and warning of falling "from" grace. departing from the faith, making Christ of no effect, to no profit, drawing back to perdition, etc.
Most of such texts in full (though provided for you before):
And because ye are sons [Paul's audience], God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father. (Galatians 4:6)... Stand fast therefore in the liberty wherewith Christ hath made us free, and be not entangled again [those who were set free] with the yoke of bondage [going back into what they were set free from]. Behold, I Paul say unto you, that if [submission to Judaizers these believers are being warned against] ye be circumcised [signifying justification obtained by keeping all the law], Christ shall profit you nothing. For I testify again to every man that is circumcised, that he is a debtor to do the whole law. Christ is become [an effectual change due to their choice] of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace[their former state]. For we [those who continue to believe] through the Spirit wait for the hope of righteousness by faith. (Galatians 5:1-5)
Take heed, brethren, [contextually not a general sense, but as believers] lest there be in any of you an evil heart of unbelief, in departing from the living God [a departure from their former state s believers]. But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin [a change of heart]. For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end [[perseverance of the saints]. (Hebrews 3:12-14)
Let us hold fast the profession of our faith without wavering [continued exhortation to persevere]; (for he is faithful that promised
And let us consider one another to provoke unto love and to good works: Not forsaking the assembling of ourselves together, [indicating departing from the faith, as follows] as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching. (Hebrews 10:23-25)
For if [a choice brethren are warned against] we [not just you] sin wilfully [not out of weakness, but with full consent, as a decision not to continue but to depart, signified by giving up being with the believers] after that we have received the knowledge of the truth [a term which refers to believing, as per 1Tim. 2:4; 2Tim. 3:7], there remaineth no more sacrifice for sins [cf. Hebrews 6:6-8; a terminal condition of judgment, with no provision for repentance and forgiveness, having forfeited what saving faith obtained], But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries. (Hebrews 10:26-27)
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 [a past condition, appropriated by faith, now forfeited by a definite denial of the same] , an unholy thing, and hath done despite unto the Spirit of grace? (Hebrews 10:28-29)
Cast not away therefore your confidence, [the issue being faith, out of which obedience flows] which hath great recompence of reward. For ye have need of patience, that, after ye have done the will of God, ye might receive the promise [not that doing earns the promise, but that saving faith is that which finally perseveres, which faith as manifested in works God - who gave faith and the ability and motivation to obey - rewards under grace] For yet a little while, and he that shall come will come, and will not tarry.
Now the just shall live by faith [the just live by faith, not merely profess it, with such faith being what appropriates justification]: but if any man draw back [a denial of justifying faith], my soul shall have no pleasure in him [as solemnly, fearfully described above]. But we [who do not draw back, but persevere] are not of them who draw back unto perdition [contextually a terminal condition of judgment, with no provision for repentance and forgiveness due to treating Christ with contempt] but of them that believe to the saving of the soul. (Hebrews 10:35-39)
But when we are judged, we are chastened of the Lord, that we should not be condemned with the world. (1 Corinthians 11:32)
For verily, when we were with you, we told you before that we should suffer tribulation; even as it came to pass, and ye know. For this cause, when I could no longer forbear, I sent to know your faith, lest by some means the tempter have tempted you, and our labour be in vain. (1 Thessalonians 3:4-5)
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 [katechō: hold fast: cf. 1Ths. 5:21] what I preached unto you, unless ye have believed in vain. (1 Corinthians 15:1-2)