Where a tradition has a claim to make about the nature of the divine, one is usually the number it makes it in, and the claim is almost always about indivisibility rather than about quantity.
The Shema, the central declaration of Jewish prayer, ends on the word one, and the Hebrew letter aleph that opens the alphabet carries the value one as well as the sound. Islam builds its whole theology on tawhid, the oneness of God, stated in the first half of the shahada and expanded in the short sura of sincerity; the corresponding sin is shirk, the association of anything else with God, which is a numerical fault before it is a moral one. Christian trinitarian thought spends centuries on the problem of saying three and one at once, and the councils that settle it are arguing about arithmetic in a way they would not have needed to if one had been an ordinary number.
Outside the monotheisms the emphasis shifts from unity to source. The Tao Te Ching has the Dao give birth to one, one give birth to two, two to three, and three to the ten thousand things, so that one is a stage of emanation rather than the end of the account. Plotinus places the One beyond being altogether, so that nothing at all can properly be said of it, and the Neoplatonic tradition after him treats every further thing as a falling away from that.
The Greek arithmological writers, for the same reason, refuse to count the monad among the numbers. It is what number is made from and what number is measured against, neither odd nor even but the principle of both.