Superficially, the theme of this poem is religion and the loss of religion. On a deeper level, however, it is about coming of age; although the young Zimmer defined this moment as the point at which he "lost religion," as the title suggests, this is not the whole truth. On the contrary, the central idea of the poem is that, while children might imagine Jesus as a "bully" in the schoolyard and associate religion with the trappings of churchgoing, Latin, and the devil, it is only through stepping away from and rejecting these things that we can actually achieve a deeper understanding of God.
The young Zimmer has determined that he will test God by deliberately avoiding Mass. When nothing happens to him and no punishment is meted out, the child Zimmer imagines that this is because God knows Zimmer is "ready for Him now." From the adult perspective of the narrative, however, we can understand that this is not really what it means to be "ready" for God. Rather, through the act of rejecting the trappings of religion, the child Zimmer has come to understand that God is not really about punishment, Latin, or the devil; God is not a "bully" waiting in the wings to judge us for minor misdemeanors. Only through testing and recognizing this has Zimmer truly become "ready for him": able to move on to a new stage of religious understanding with an appreciation of God as more than an Old Testament rule-enforcer.