The "true faith" was found when stained glass, statues and other artwork told the story. It had been told wrong by peasant preachers. The new teller swept the carpenter's dust off the floor.
Miracle (Christ) began to have playtime with a mother who was sitting for elaborate portraits and chryselephantine statues. She sewed him fancy trousers while sitting for these portraits. Noah's flood never reached the towers in Babylon in which they sat. There, he lived a wild infancy.
Even the title of this poem is ironic. The very term "true faith," in the context of a collection that is partly set during times of civil war, seems silly, even dangerous. The irony becomes manifest in that while replacing the more rustic forms of Christianity, the church officials sweep out the dust, thereby separating themselves from the truth about the religion.
The second section of this poem (the turning point is at the line beginning "miracle") reasons backward in time to see what religion would have looked like. The result is an incongruous picture of a stiff mother Mary who is posing for portraits rather than living the life of a real person. The irony is that this version of "wisdom" abandons the basic humanity in what claims to be a humane religion.