"Even lowly serfs knew the doctrinal points in the Creed".  They RECITED them on Sundays.  Just as tots can be taught to spell word like M I S S I S S I P P I----but not know the difference between vowels and consonants, syllables, or even what Mississippi is.  The lowly serfs didn't know 'doctrinal points'.  One of the reasons why Guilds had Morality Plays---was to teach the people about Christian living.  The reason why churches/cathedrals had stained glass windows depicting scenes from Christ's life/saints etc. was to teach the people about the life of Christ---otherwise they would not know about it.