Godwins Law - well that's us told then !
were it not that Christianity is sheltered in the strong arms of science, the science against which it had vainly struggled, the civilization of modern Europe might fall, as fell the civilization of ancient Rome.
read it again
it is an if ... then clause
if clause ( er protasis) is subjunctive
then clause (apodosis) conditional
so this makes it a closed conditional
that is - If X then Y but it wont ....
if Christianity didnt have machine guns(*) then it might fall ( but it wont because it does) - so the prophesy is er the other way round. Think Trump and negatives
(*) I did wonder about Ch-Ch and Islam since he had been present at the massacre of Omdurman ( 5000 dead on one side and 6 dead on the other - can you tell which one is which?)
and I am grateful to Mr Heyer - classics master - and niece to Georgette heyer who taught me sequence of tenses in conditional clauses in 1966.
( much more difficult in NY Greek where it is easier to just learn the examples)