only way to get devotion skill after all blue masks have been converted is check devotion skill and put villager to polish statue (and hope that he/she gains devotion) and having JOAT (like retired chief) teaching at nursery so some kids can get devotion skill (if they have any skills on devotion and are interested on it) and of course parentin master/adept devotees.

i have on my game alot devotees and more comes after nursing since most devotees i have and will get are females.

of course certain events give devotion skill or more blue masks.