The verse says what will happen to the action and it's result that is already established IF they don't also have the simple action of having held fast. It will end.
How is this even remotely to be considered to be some great gospel of assurance? You don't even know today if you're believing is for real because tomorrow may show it to have been fake all along. Or is there some caveat in once saved always saved that negates it's own assertion that you know without a doubt that you are really saved without knowing if you'll believe all the way to the end?
And you say 'no middle ground', yet the Galatians and the Corinthians are on record as being real believers, and they stopped believing. But it seems, once again, once saved always saved doctrine will have a way around it's own doctrine that says whoever stops believing never really believed or was saved in the first place.
God hasn't changed.
Joshua 23
8 but you shall
hold fast to the Lord your God, as you have done to this day. 9 For the Lord has [
b]driven out from before you great and strong nations; but
as for you, no one has been able to stand against you to this day. 10 One man of you shall chase a thousand, for the Lord your God
is He who fights for you, as He promised you. 11
Therefore take careful heed to yourselves, that you love the Lord your God. 12
Or else,
if indeed you do go back, and cling to the remnant of these nations—these that remain among you—and make marriages with them, and go in to them and they to you, 13
know for certain that the Lord your God will no longer drive out these nations from before you. But they shall be snares and traps to you, and scourges on your sides and thorns in your eyes,
until you perish from this good land which the Lord your God has given you.