Typically, Protestants who believe in the “once saved always saved” doctrine also believe we can “know” we have eternal life. And the knowledge here is taken to be knowledge with absolute certainty, i.e., having no possibility of error—as opposed to knowledge with moral certainty, which involves the possibility of error. They often cite 1 John 5:13 for support.
There’s a corollary that goes with the above belief—namely, that if someone falls into grave sin, then he was never really saved in the first place. This often is posed as a counter to a Catholic who, in trying to defend the idea that it’s possible to lose your salvation, cites a person whom most thought was truly a “born again” Christian and yet committed grave sin—e.g., adultery or apostasy.
It never occurred to me until recently that this corollary is incompatible with the belief that we can “know” we have eternal life.
Think about it: according to the above belief, committing a grave sin is a sign that a person was never saved in the first place. But here’s the question: do we have absolute assurance that, from this day forward, we will never commit a sin like apostasy that signifies we were never saved in the first place? The answer is no.
Now, if we don’t know whether we’re going to eventually commit such a sin, then we don’t really know with absolute assurance whether we’re really saved in the first place or not. We wouldn’t have such assurance until we die.
So, the belief that we can “know” with absolute assurance that we’re saved is logically incompatible with the belief that a grave sin committed by a “Christian” is evidence that he was never really saved in the first place.
Protestants who profess the above two beliefs must choose which belief they will keep. Which will it be? Our Protestants friends will have to decide.