The issue is the sesame-street song, "One of these things is not like the others." A rational mind categorizes rationally. We know chimps are not human for a number of discriminating factors. We know early on that wood is not metal, though we can't explain the difference between a rarefied alloy and a complex organic polymer.
Similarly, there are distinctives that limit inclusion in a religious classification. An essential element in Christianity is the Christ. The essential elements that are the minimum standard for inclusion in the set called Christian are called Essential Christianity. Without these basic elements your religious perspective, however dear to you cannot be Christian. Instead you have what we call Sheilaism, in the sociology of religion. One of these distinctives is belief in Jesus as the only birthed son of the God of the Tanankh (El Shaddai, Hashem, Elohim, etc). If you fail to believe in the Father God that engendered Jesus, or fail to believe in Jesus' divinity, you may have a faith that is organized, deeply personal and provides you with satisfying answers, but it isn't Christian, it has no Christ.
I could call myself an Afro-American, but I would be foolish. I lack the credentials. None of my wildly mixed heritage happens to include people who have ancestry in among the peoples of Western, Central, East or south Africa. Granted some were from Egypt, but no matter how you slice it Egyptians have an Asiatic origin. My grey eyes and mouse brown hair would tend to disuade belief, and my pale complexion wit ha yellowish cast would be the clincher. No one would accept that I was afro-american, because I lack the essential characteristics. My claim would be foolish.
Similarly you cannot be an A-theist and an authentic member of a Theistic religion.
