William Livingston was a Presbyterian.
He was identified as an Presbyterian by: the Library of Congress and A Worthy Company: Brief Lives of the Framers of the United States Constitution, written by M. E. Bradford. (Source: Ian Dorion, "Table of the Religious Affiliations of American Founders", 1997).
From: Robert G. Ferris (editor), Signers of the Constitution: Historic Places Commemorating the Signing of the Constitution, published by the United States Department of the Interior, National Park Service: Washington, D.C. (revised edition 1976), pages 185-186:
Livingston was born in 1723 at Albany, N.Y. His maternal grandmother reared him until he was 14, and he then spent a year with a missionary among the Mohawk Indians. He attended Yale and graduated in 1741.
...in 1745 he married Susanna French... She was to bear 13 children.
Three years later, Livingston was admitted to the bar and quickly gained a reputation as the supporter of popular causes against the more conservative factions in the city. Associated with the Calvinists in religion, he opposed the dominant Anglican leaders in the colony and wielded a sharply satirical pen in verses and broadsides. Attacking the Anglican attempt to charter and control King's College (later Columbia College and University) and the dominant De Lancey party for its Anglican sympathies, by 1758 Livingston had risen to the leadership of this faction. For a decade, it controlled the colonial assembly and fought against Parliamentary interference in the colony's affairs. During this time, in 1759-61, Livingston sat in the assembly...
Livingston died at Liberty Hall in his 67th year in 1790. He was originally buried at the local Presbyterian Churchyard, but a year later his remains were moved to a vault his son owned at Trinity Churchyard in Manhattan; and in 1844 were again relocated, to Brooklyn's Greenwood 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.