After giving a parish seminar, I was approached by a Fundamentalist who said he believed in an absolute assurance of salvation: accept Christ as Lord and Savior, and your salvation is a sure thing. I posed a hypothetical to him:
Let's say your minister became a "born-again Christian" at the age of 15. Now he is 75. In the intervening 60 years the man has lived an exemplary life. So far as he or anyone else knows, he never has committed a single serious sin. But then, at age 75, comes a change. During one horrible day, he robs a bank, deliberately runs over a cat with his car, commits adultery, blasphemes loudly in public, murders a neighbor, and then commits suicide, dying unrepentant.
My question to the Fundamentalist: Does your minister go to heaven or hell? "To hell, of course," he replied. "No one committing such sins and not repenting of them would go to heaven."
"But I noted that the minister was a born-again Christian, which means, by your principles, that he should be guaranteed entrance to heaven."
"He wasn't born again."
"Yes, he was, as I stated at the beginning."
"No, he wasn't," the Fundamentalist said in an insistent tone.
"Now wait a minute," I answered. "This is my hypothetical. I set the parameters, and I say he was a born-again Christian."
"He couldn't have been. No born-again Christian would commit such sins."
For a moment I didn't know what to reply. Then it occurred to me: "What you are saying is perilously close to the Catholic position, though you don't realize it. You are saying that we can't tell whether someone really is born again until he's safely dead. We might think, for years, that someone is born again, but his actions late in life might disprove our assumption.
"This is true even in the case of someone, like the minister, who himself thought he was a born-again Christian. Your argument must be that he was mistaken, as shown by his later sins. The conclusion is that between the ages of 15 and 75 he could not have had a true absolute assurance of salvation, because such an assurance is not available until one's final moments.
"This is very like the Catholic teaching, which holds that we can have a moral assurance of salvation but not an absolute assurance. We can have confidence that we will go to heaven if we now are in the state of sanctifying grace and realize that God gives us enough helping graces to remain in sanctifying grace, but we can't have an absolute assurance of salvation because, knowing our weaknesses, we know that we might sin grievously in the future, forfeiting everything."
Homily on Assurance of Salvation
See also: Cloud of Witnesses: A Biblical Primer on the Communion of Saints
Catholic Home Study Resources
Mary's Vow of Virginity
Follow-up question from Arron: "If you were brought up away from all churches, chapels, popes and bishops, and you only had the Bible as your ONE and ONLY source of information. Would you honestly believe Mary was a sinner or not?"