Abraham Clark 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: 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 92:
When Congress adjourned, in June, 1794, Mr. Clark retired from public life; and early in the autumn of that year, he died of inflammation of the brain, (caused by a coup de soliel, or "stroke of the sun,") in the sixty-ninth year of his age. He was buried in the church-yard, at Rahway, New Jersey.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 48:
Mr. Clark was a warm partisan, and his feellings of attachment or repulsion were very strong. He had witnessed so much of the cruelty and oppressions of Great Britain, that his feelings of hatred could not be soothed by the treaty of peace, although he patriotically acquiesced in whatever tended to his country's good.
Clark was stricken with a sunstroke in 1794 at his birthplace in Roselle, where he had lived all his life... He died a few hours later, at the age of 68, in the narby town of Rahway and was buried there in the Presbyterian Cemetery.