This is a very powerful poem about a turning point in the life of the narrator, the titular "Zimmer." Written from the perspective of an adult remembering, and parsing, his childhood experiences, its theme is one of religious maturity. As a child, Zimmer, having determined "on purpose" to test God by not attending Mass, moves away from the idea of Jesus as a "playground bully" who will punish him for failing to adhere to the trappings of organized religion, so he actually becomes "ready for" Jesus. As a child, the failure of Jesus to appear and "pound" him for his "irreligious" behavior may have struck Zimmer as indicative that he was "grown up" and unlikely to respond to that kind of punishment. As an adult, however, the poet recognizes that this moment was a turning point on a wider level: he was "ready" now, having lost religion and yet having gained a deeper understanding of what God and Jesus really meant.
To the child Zimmer, the one who ostensibly had not "lost" religion, the idea of God was a distant one, God "a one-eyed triangle" in the sky whose purpose was to punish. The poem is laced with technical, clerical language which indicates that the young Zimmer was a regular churchgoer, understanding the superficial purpose of the "thurible," mumbling Latin "cassocked and surpliced." And yet, this child who was so thoroughly steeped in church religion had no understanding of what God really was, imagining religion as "the devil roaring" at the "irreligious" and Christ as a "playground bully." This association, the adult Zimmer implies, is a childish one: there is a connection drawn between "venial sins" and "the schoolyard," as if only a child should believe his religion is really grounded in strict rules and the threat of punishment. Having once stepped away from Mass and found that he was not punished, the child, far from becoming "irreligious," in fact takes his first step towards an adult relationship with God. He is "ready for him now" because that association has been broken and it is possible for Zimmer to begin understanding Jesus as something other than a threat.