I have no problem with a person instantaneously becoming a disciple of Christ. However, that is but entering the door of salvation. Salvation itself is a life-long process, and perhaps extends even beyond death. For at death, many or perhaps all disciples of Christ have not yet been fully delivered from sin. They may retain wrong attitudes, a condemning spirit, hateful feelings, destructive desires, and much else. God will never give up on anyone until their natures are fully refined. He will do whatever it takes to bring this about.
Christ’s death is not about forgiving people. Christ forgave people prior to His death. The purpose of Christ’s death is to deliver people from wrongdoing, to begin the process of refining their natures, and to complete it with their coöperation. If we think we can receive the grace of God without coöperating with it, we receive it in vain.
… We implore you on behalf of Christ, be reconciled to God. For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God. Working together with him, then, we appeal to you not to receive the grace of God in vain. (2 Cor 5:20-6:1 ESV)
If we examine the passages that give us the purpose of Christ’s death, we will see that they are not about forgiveness of sin, but about deliverance from sin (yes, even the abolition of sin!):
*I Peter 2:24 He himself endured our sins in his body on the tree, that we might die to sin and live to righteousness. By his wounds you have been healed.
II Corinthians 5:15 And he died for all, that those who live might live no longer for themselves but for him who for their sake died and was raised.
Romans 14:9 For to this end Christ died and lived again, that he might be Lord both of the dead and of the living.
Titus 2:14 who gave himself for us to redeem us from all iniquity and to purify for himself a people of his own who are zealous for good deeds.
Heb 9:26 …he has appeared once for all at the end of the age for the abolition of sin by the sacrifice of himself.*
So we see from all of these passages that Christ’s death for our benefit, is not to forgive our sins, but to deliver us from sin, and that that deliverance does not take place instantaneously, but is a process and requires our continual coöperation.
By the way, the Eastern Orthodox position on salvation is the same as mine— that salvation is a process. Here is a 10 minute video in which Metropolitan Kallistos was confronted by the question, “Are you saved?” and in which he shares the answer that he gave to the questioner, an answer that indicates that his salvation was not an accomplished fact, but rather is a process that Christ is accomplishing in Him.