This pre-forgiveness of future sins pivots around the belief that one can be free from sin whilst in bondage to it. Of course the idea of freedom from sin is simply freedom on a theoretical/judicial basis with no real transformation of the believer's life, and they still remain slaves to their sin nature. It is the same lie that Satan orchestrated in the beginning: "ye shall not surely die". Basically, according to this doctrine, a believer can live in opposition to God and not die spiritually because they have been forgiven of future sins at initial conversion.
It is a licentious doctrine that conflicts with sanctification of believers and hinders spiritual growth. It devalues of the holiness of God and His will that His people are holy in this present world and it allows for believers to live and die with unrepentant sins.
The atonement of Christ covers all sins however the moment a sinner repents and believes in the Lord Jesus they will be forgiven for sins committed up to the point of conversion, the condemnation upon them because of their past transgressions will be taken away, and they will be declared righteous in God's sight by grace. Their desire now is to stop yielding to the dictates of the flesh and live a holy life, which God enables them through His indwelling Spirit. Rom 6:6- Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.
Obedience, repentance, sanctification, holiness would all be meaningless if future sins are forgiven before committed. Nowhere does the scripture allude to such doctrine, rather we have passages like these:
Prov 28:13- He that covereth his sins shall not prosper: but whoso confesseth and forsaketh them shall have mercy.
Titus 2:11-12- For the grace of God that bringeth salvation hath appeared to all men, Teaching us that, denying ungodliness and worldly lusts, we should live soberly, righteously, and godly, in this present world; The grace that saved us also teaches us to deny ungodliness and worldly lusts. We would not need to guard against sin after salvation if future sins are forgiven before committed because we would still be in a justified state while indulging them.
1 John 1:19- If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. We would not need to confess our sins to God when we sin because we would still be in right standing with Him. In fact, God would not chasten anyone for future sins if they were forgiven at initial conversion because when He forgives He forgets them (Heb 8:12), yet there are instances of backsliding and chastening. Therefore chastening implies that either the individual was never purged, cleansed, and forgiven for future sins at conversion or the sins were reinstated into their account.
2 Peter 1:9- But he that lacketh these things is blind, and cannot see afar off, and hath forgotten that he was purged from his old sins. The blood of Christ purges the sinner's old, past, sins.
As believers we are to repent and seek the mercy of God when we sin and have been convicted by the Holy Spirit for His grace is always available to cleanse and pardon. Unrepentant sin defiles believers' spiritual garments and grieves the Holy Spirit and the bible is clear that the unrighteous will not inherit the kingdom of God.
No one can sin their way into Heaven for God is of purer eyes than to behold evil. Those on the narrow way must be dead to sinful pleasures, dead to self, dead to idolatry, and alive to God, which is possible through God's superabundant grace. The sad reality is that some Christians who think they are walking on the narrow way are actually gallivanting with multitudes on the broad way to destruction.
Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind, Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God. And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God (1 Cor 6:9-11).
Follow peace with all men, and holiness, without which no man shall see the Lord (Heb 12:14).
Thou hast a few names even in Sardis which have not defiled their garments; and they shall walk with me in white:for they are worthy. 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 3:4-5). Sins committed after salvation would not defile our spiritual garment if they have been forgiven in advance.