I'm grown a man no doubt, I've broken bounds: You should not take a fellow eight years old And make him swear to never kiss the girls. (223-225)
Fra Lippo is pretty explicit here about how the Church has made its adherents swear vows that are unnatural and that butt up against simple human biology. He's not able to suppress these impulses, even with the full weight of the Church and the potential consequences of sin hanging over him. His individuality is much more important to him here.
Old Aunt Lapaccia trussed me with one hand, (Its fellow was a stinger as I knew) And so along the wall, over the bridge, By the straight cut to the convent. Six words there, While I stood munching my first bread that month: "So, boy, you're minded," quoth the good fat father Wiping his own mouth, 'twas refection-time,— "To quit this very miserable world? Will you renounce" . . . "the mouthful of bread?" thought I; (88-96)
Lest we think that all members of the clergy—and in particular those who enter formal orders (such as monks and friars)—are sincere in their vows of obedience, chastity (which basically means no hanky-panky), and silence (sometimes), that's not necessary how the cookie crumbles. Sometimes people (like Fra Lippo) take vows because they are hungry. To an eight-year-old, those vows compared to an immediately full belly really doesn't warrant that much contemplation.
Do you feel thankful, ay or no, For this fair town's face, yonder river's line, The mountain round it and the sky above, Much more the figures of man, woman, child, These are the frame to? What's it all about? To be passed over, despised? or dwelt upon, Wondered at? oh, this last of course!—you say. But why not do as well as say,—paint these Just as they are, careless what comes of it? God's works—paint any one, and count it crime To let a truth slip. (286-296)
Here, Lippo speaks to his new friend, the night watchman, and he's questioning him about the potential of art. Lippo wants to present things as they are. After all, they are all God's works and convey certain truths. The Church, he implies here, wants to cover this aspect up, lest "a truth slip" (one that is not authorized by the Church). This is the conflict throughout the poem between the Church's wishes to idealize the soul through art, and Lippo's drive to tell the truth and convey the beauty of everything he sees—in the hopes it will make the audience appreciate it and get something intellectually satisfying out of it.
Your business is not to catch men with show, With homage to the perishable clay, But life them over it, ignore it all, Make them forget there's such a thing as flesh. Your business is to paint the souls of men. (183-187)
The Church looked at the job of painting as to somehow capture the souls of men. Art's only function is to direct the human gaze higher, to the spiritual plane, that heavenly realm that awaits people after death. The flesh is unimportant.
And my whole soul revolves, the cup runs over, The world and life's too big to pass for a dream, And I do these wild things in sheer despite, And play the fooleries you catch me at, In pure rage! (250-254)
Having to suppress his true desires—to paint what he wants to in order to convey the true beauty of human physicality and the everyday world—takes a toll on Bro Lippo. He rages, and pretends to play "fooleries," all because he's being forced to pass by the world in order to focus his eyes on the "dream" (the spiritual afterlife). He'd much rather be recording the beauty of the world and its immensity.