The words of the title come from the concluding praises—called a doxology—that sometimes follow the saying of the Our Father. It goes, "For thine is the kingdom, the power and the glory, now and forever." For all the controversy the novel caused among the powers that be in the Church, the story is rather hopeful for the victory of Catholicism. The arrival of the new priest at the end suggests that the Church will go on—the gates of Hell will not prevail against it, as the Bible says. God's power and glory will win—they don't call him God for nothing.
To be sure, the novel doesn't prove the truth of Catholicism. It's not meant to persuade or convert readers. However, it is something of a response to the persecution of Christianity wherever it happens. Greene seems to say that no matter how effective the suppression or how wicked the religious leaders, the Church will survive—as long as it knows how to love…