The word 'ideology,' which is now common, means nearly the same thing as was formerly meant by 'creed.' An 'ideology' may be defined as a system of beliefs leading to a line of conduct, both public and private, and supported, whenever it is politically important, by a priesthood or something analogous. The word was brought into general use by Napoleon, ... I shall use the word 'ideology' as practically synonymous with the word 'creed', but with slightly less implication of dogmatic definiteness.
Source: New Hopes for Changing World, 1951, Part II, chap. 13:
Creeds and deologies.