The greatest of all sins is the presumption of salvation, what we Catholics call Sin Against the Holy Ghost. It is unforgivable.
It dismisses the HOPE in the THREE theological virtues. Actually it dismisses the other two as well when you think about it.
1 John 5:13, "These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God."
1 Peter 1:3 Blessed be the God and Father of our Lord Jesus Christ, which according to his abundant mercy hath begotten us again unto a lively hope by the resurrection of Jesus Christ from the dead, 4 To an inheritance incorruptible, and undefiled, and that fadeth not away, reserved in heaven for you, 5 Who are kept by the power of God through faith unto salvation ready to be revealed in the last time.
We are kept by the power of God. Not by our own power or effort. You do believe He is able don't you?
“The greatest of all sins is the presumption of salvation”
As for those who have accepted Christ as their Lord and Savior........
Galatians 2:21:
“I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain.”