Thomas McKean was a Presbyterian.
He was identified as a Presbyterian by A History of Delaware Through its Governors 1776-1984 by Roger A. Martin. (Source: Ian Dorion, "Table of the Religious Affiliations of American Founders", 1997).
From: B. J. Lossing, Signers of the Declaration of Independence, George F. Cooledge & Brother: New York (1848) [reprinted in Lives of the Signers of the Declaration of Independence, WallBuilder Press: Aledo, Texas (1995)], page 141-142:
Thomas M'Kean was born in New London, Chester County, Pennsylvania, in the year 1734. His father was a native of Ireland, and Thomas was the second child of his parents. After receiving the usual elementary instruction, he was placed under the care of the Reverend Doctor Allison, and was a pupil under him with George Read. At the conclusion of his studies, he entered the office of David Finney, of New Casstle, as a law student...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), page 102:
McKean lived out his life quietly in PHiladelphia. He died in 1817 at the age of 83, survived by his second wife and four of the 11 children from his two marriages. He was buried in Laurel Hill Cemetery.