One of the most pernicious doctrines is “once saved always saved”.
It was always known that serious/mortal sin cut a soul off from God and life.
Many holding to OSAS develop rationalisations of sin and become mired in sin, they don’t treat it seriously. “ I’m saved by the blood already, I’ll just say sorry and move on “ won’t cut it with mortal sin. They compound their sin and blindness.
Mortal sins require confession and absolution from an Apostolic Successor as Jesus ministers and ambassadors. Mortal sin estranges a soul from God and requires Christ’s ambassadors to reestablish the relationship, that’s their job.
The breath of Life Jesus imparted to The Apostles for the forgiveness of sin is imparted to sinners who have died because of mortal sin.
“Again Jesus said, “Peace be with you! As the Father has sent me, I am sending you.”22 And with that he breathed on them and said, “Receive the Holy Spirit. 23 If you forgive anyone’s sins, their sins are forgiven; if you do not forgive them, they are not forgiven.” John 20:21-23
Jesus sent the Apostles as the Father had sent Him with power to forgive sin.
"Truly, I say to you, in the new world, when the Son of Man will sit on his glorious throne, you who have followed me will also sit on twelve thrones, judging the twelve tribes of Israel.“
The Apostles in their successors do judge the faithful through ministry of reconciliation, that by confession, they judge forgiven or not forgiven.
“In addition to these there is also a seventh, albeit hard and laborious: the remission of sins through penance…when he does not shrink from declaring his sin to a priest of the Lord.” Origen, Homilies on Leviticus, 2:4 (A.D. 248).
‘God is not mocked.’ God cannot be mocked, nor deceived, nor deluded by any deceptive cunning. Yea, he sins the more, who, thinking that God is like man, believes that he evades the penalty of his crime if he has not openly admitted his crime…I entreat you, beloved brethren, that each one should confess his own sin, while he who has sinned is still in this world, while his confession may be received, while the satisfaction and remission made by the priests are pleasing to the Lord?” Cyprian, To the Lapsed, 28-29 (A.D. 251).
“These are capital sins, brethren, these are mortal.” Pacian of Barcelona, Penance, 4 (A.D. 385).
“For if any one will consider how great a thing it is for one, being a man, and compassed with flesh and blood, to be enabled to draw nigh to that blessed and pure nature, he will then clearly see what great honor the grace of the Spirit has vouchsafed to priests; since by their agency these rites are celebrated, and others nowise inferior to these both in respect of our dignity and our salvation. For they who inhabit the earth and make their abode there are entrusted with the administration of things which are in Heaven, and have received an authority which God has not given to angels or archangels. For it has not been said to them, ‘Whatsoever ye shall bind on earth shall be bound in Heaven, and whatsoever ye shall loose on earth shall be loosed in Heaven.’ They who rule on earth have indeed authority to bind, but only the body: whereas this binding lays hold of the soul and penetrates the heavens; and what priests do here below God ratifies above, and the Master confirms the sentence of his servants. For indeed what is it but all manner of heavenly authority which He has given them when He says, ‘Whose sins ye remit they are remitted, and whose sins ye retain they are retained?’ What authority could be greater than this? ‘The Father hath committed all judgment to the Son?’ But I see it all put into the hands of these men by the Son.” John Chrysostom, The Priesthood, 3:5 (A.D. 387).
“All mortal sins are to be submitted to the keys of the Church and all can be forgiven; but recourse to these keys is the only, the necessary, and the certain way to forgiveness. Unless those who are guilty of grievous sin have recourse to the power of the keys, they cannot hope for eternal salvation. Open your lips, them, and confess your sins to the priest. Confession alone is the true gate to Heaven.” Augustine, Christian Combat (A.D. 397).
“Just as in the Old Testament the priest makes the leper clean or unclean, so in the New Testament the bishop and presbyter binds or looses not those who are innocent or guilty, but by reason of their office, when they have heard various kinds of sins, they know who is to be bound and who loosed.” Jerome, Commentary on Matthew, 3:16,19 (A.D. 398).
Binding and loosing Authority means that Jesus confirms in Heaven the judgement of His Apostolic servants on Earth, Jesus granted Authority to the Apostolic servants to forgive sin in His Name.
This is a great joy and gift to the Church, that those who have committed mortal sins can be restored to life and Grace.
They don’t arrive at Judgement without already having received the Apostolic Judgement of His ambassadors on Earth.