William Floyd was a Presbyterian.
He was identified as a Presbyterian by the Presbyterian Historical Society and the Presbyterian Church, USA. (Source: Ian Dorion, "Table of the Religious Affiliations of American Founders", 1997).
From: Robert G. Ferris (editor), Signers of the Declaration: Historic Places Commemorating the Signing of the Declaration of Independence, published by the United States Department of the Interior, National Park Service: Washington, D.C. (revised edition 1975), pages 53-54:
In 1803, in his late sixties, at a time when most men possess lesser ambitions, Floyd deeded his Long Island home and farm to his son Nicoll, and set out with the rest of his family to make a new life on the frontier. During the first year, he built a home at present Westernville, N.Y. There he succumbed, at the age of 86 in 1821, and was buried int he Presbyterian Cemetery.Note that numerous sources and authoritative references have been consulted in order to ascertain the religious affiliation of the American Founding Fathers. Note that the excerpts and references mentioned on this page are not the only references used in order to identify this person's religious affiliation.