Check out this post by Jim Peron on James Madison's religion. We see things pretty much eye-to-eye. The bottom line on Madison is that while he may have briefly flirted with orthodox Christianity when he was at Princeton, soon after he came home he abandoned that creed for what I would call theistic rationalism. David Holmes of William and Mary uses the term "moderate deism," and James H. Hutson (whose paper Peron references) speaks of "the many mansions of deism" to describe Madison's creed. In other words his creed was deist like, but not strict deism. Theistic rationalism could be seen as a broader form of deism, but also a more liberal form of Christianity (hence Holmes' term "Christian-Deism," as opposed to the "non-Christian-Deism" of Thomas Paine and Ethan Allen). But strictly speaking, arguably it is neither deism nor Christianity, hence the need for a new term.
Certainly Madison began adulthood as an orthodox Christian. Shortly after his graduation from, what would become Princeton, he wrote a friend suggesting that their generation become “fervent advocates in the cause of Christ.” But as historian James Hutson, for the Library of Congress, noted: “Two months later Madison renounced his spiritual prospects and began the study of law... For the rest of his life there is no mention in his writings of Jesus Christ nor of any of the issues that might concern practicing Christians. Late in retirement there are a few enigmatic references to religion, but nothing else.”
Biographer Irving Brant quoted Rev. Alexander Balmaine regarding Madison. Balmaine was married to one of Madison’s cousins and was the minister who performed his marriage ceremony. And Balmaine said that Madison’s “religious feeling, however seems to have been short lived. His political associates were those of infidel principles, of whom there were many in his day, if they did not actually change his creed, yet subjected him to a general suspicion of it.” In addition Brant quotes the local Episcopal Bishop, William Meade, who recounted that a conversation with Madison took an unexpected turn, at Madison’s instigation. The comments made by Madison, wrote the Bishop, “left the impression on my mind that his creed was not strictly regulated by the Bible.” And Brant quoted a gentleman who had dinner with Madison. Madison queried the man about “how the cause of liberal Christianity stood with us, and if the Athanasian creed was well received by our Episcopalians. He pretty distinctly intimated to me his own regard for the Unitarian doctrines.”
Bishop Meade said that whatever “may have been the private sentiments of Mr Madison on the subject of religion, he was never known to declare any hostility to it.” He, like the deist Washington, attended a local church and invited ministers to his home but “did not kneel himself at prayers.” And Hutson notes that Madison went for long stretches of time without bothering himself with any church service. And at one point he told the governor of Vermont, with Jefferson standing there, that he had not attended church for “several years”.