John F. Mercer was an Episcopalian.
From: Political Graveyard website (http://politicalgraveyard.com/bio/mercer.html#R9M0J6I6Q; viewed 7 December 2005):
Mercer, John Francis (1759-1821) - Born in Stafford County, Va., May 17, 1759. Brother of James Mercer. Democrat. Colonel in the Continental Army during the Revolutionary War; Delegate to Continental Congress from Virginia, 1783-84; member, U.S. Constitutional Convention, 1787; member of Maryland state house of delegates, 1788-92, 1800-06; U.S. Representative from Maryland, 1792-94 (at-large 1792-93, 2nd District 1793-94); Governor of Maryland, 1801-03. Episcopalian. Member, Freemasons. Died in Philadelphia, Philadelphia County, Pa., August 30, 1821. Interment at a private or family graveyard, Anne Arundel County, Md. Mercer County, Mo. is named for him.
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.