Originally posted by @rajk999
Jesus is the final judge of these matters.
The ideal situation is one where a Christian convert is able to resist all evil.
The worst situation is one where the sinning is unforgivable.
Of course there are a zillion in-betweens.
OSAS applies to a particular class of people identified by Christ.
It cannot be applied across the board to all Christians.
Some simple reasoning explains two doctrines.
In hard-line Christianity, the doctrines of unforgivable sin and extreme eternal punishment (I'll call it Hell here) go hand in hand. Due to them, the possibility that God's mercy will result in our sinful enemies avoiding or short-cutting cosmic justice is eliminated. The sins that are deemed unforgivable are so because they are against God (or one member of the godhead) because otherwise, if the sins are against us, we might be induced to "forgive and forget." But how can we forgive a sin against God that even He cannot forgive?
These two doctrines rise or fall together. Without one, the other becomes vulnerable to a particular criticism, that being, if it is logically possible for God to be mercifully forgiving to all sinners for all sins, and God can do whatever is logically possible, why will there ever be anyone in Hell?
Ipso facto, Hell would not need to exist. That wouldn't do at all, would it? So these two doctrines necessitate one another just for the Hell of it.