The sin of unbelief is the sin unto death.
That is the only sin that the intercessory blood of Christ does not instantly atone for the believer (now ex-believer).
Any and all other sins a believer commits are instantly atoned for by the blood of Christ, continually interceding for them on the altar in heaven, that they believe and trust in.
The continued believing of the believer keeps them safe in the declaration of righteousness and righteous perfection reckoned to them the moment they first believed. Only deliberate and willful unbelief in an abandoning of the sacrifice of Christ for the forgiveness of sin can alienate the believer from that forgiveness. The believing person remains justified and perfect before God as long as he keeps believing.
Catholics, on the other hand are continually getting re-justified because they do not believe that they were reckoned righteousness and made perfect one time for all time, not needing to be repeated, by the blood of Christ when they first believed. They are continually working deeds and riruals in faith to achieve that declaration of righteousness. And so they never achieve a position of being saved in this life.
For the Catholic it can only be determined that you are righteous, and therefore, declared righteous (justified/saved), until you see if you endure to the end in the faithful and satisfactory performance of righteous rituals and deeds. It's a works justification gospel. The one condemned in scripture.