1

The meaning of doctrine is set of beliefs

So, I think, anyone would hardly use the word 'doctrines'

for e.g: I could never 100% agree with the Catholic doctrine.

2

Doctrine is a normal singular noun with a regular plural. The Catholic church has multiple doctrines - notionally distinct sets of beliefs. There is the doctrine of the Virgin Birth, The doctrine of the Trinity, the doctrine of the Resurrection, and others.

Similarly a "herd" is a group of cows. This does not mean that the plural "herds" is impossible to use. There may be multiple groups of cows.

You can say "I could not agree with Catholic doctrine" In this context you change your perspective and group all the doctrines mentioned above into one. This is no more confusing that a farmer who has cows in two fields sometimes thinking of them as two herds, and sometimes as one herd.

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