Množina: doctrines

ETYM French doctrine, Latin doctrina, from doctor. Related to Doctor.
A belief (or system of beliefs) accepted as authoritative by some group or school; SYN. philosophy, school of thought, ism.
The official teaching of a particular religion. For example, in Christianity, the doctrine of the Trinity does not appear in the Bible, but was developed in thought and debate, defined by church councils, and expressed in the creeds. Different branches of a faith may have slightly different doctrines; for example, different Christian groups hold different doctrines about the Eucharist.

Singular of doctrines is doctrine.