As a clergyman, Dr. Chasuble is a natural target for the irreverent Oscar Wilde. The playwright has already lampooned the Victorian Virtues of beauty, youth, fashion, social ascendance, and education... he isn’t going to leave out religious piety. Chasuble is willing to christen the two wannabe-Ernests with no questions asked. Christening is a sacrament, usually meaning "sacred," but Chasuble just seems happy to have the business.
Chasuble and Miss Prism are pretty much male and female versions of the same character: stuffy, pedantic, and celibate. Their flirtations echo each other:
Were I fortunate enough to be Miss Prism's pupil, I would hang upon her lips. [Miss Prism glares.] I spoke metaphorically. —My metaphor was drawn from bees. Ahem! (II.26)
Chasuble conceals his romantic signals to Miss Prism beneath silly, scholarly figures of speech, just as Miss Prism does later with her metaphor drawn from fruits. All this subterranean flirting pays off at the end of the play. Infected by the romantic atmosphere, Chasuble embraces his "Laetitia" and piles on an implied third engagement to the resolution.