The speaker of "Soliloquy of the Spanish Cloister" describes the supposed sins of Brother Lawrence in great detail, showing so much expertise in these various sins that it's clear he really knows what he's talking about! The speaker is guilty of many of the sins of which he accuses Brother Lawrence, despite his frequent claims of religious and moral superiority. Robert Browning is famous for allowing the speakers of his dramatic monologues to condemn themselves – they expose their own sins and vices through their own speech, rather than through a narrator's description. The speaker of the "Soliloquy" is no exception: he inadvertently shows himself to be a religious hypocrite.
The speaker exposes his own hypocrisy most obviously through the parenthetical asides at line 32, where he admits that Brother Lawrence does not actually show the sinful lust that the speaker has accused him of.
The speaker's religious hypocrisy is even evident in the poem's form: the syntactic breaks ("G-r-r-r!") and the irregularities of meter indicate his loss of control.