Thomas Mifflin was a Quaker and a Lutheran.
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 193-194:
Merchant-politician-soldier Mifflin lived in affluence for most of his years, but died in poverty. He had forsaken his Quaker faith to fight in the War for Independence. Later, he served as President of the Continental Congress and as Governor of Pennsylvania.
A member of the fourth generation of a Pennsylvania Quaker family who had emigrated from England, Mifflin was born at Philadelphia in 1744, the son of a rich merchant and local politician. He studied ast a Quaker school and then at the College of Philadelphia (later part of the University of Pennsylvania)... In 1774 he attended the Continental Congress (1744-76). Meanwhile, he had helped to raise troops and in May 1775 won appointment as a major in the Continental Army, which caused him to be expelled from his Quaker faith. In the summer of 1775 he first became an aide-de-camp to Washington and then Quatermaster General of the Continental Army. Late in 1775 he became a colonel and in May 1776 a brigadier general. Preferring action to administration, after a time he began to perform his quartermaster duties perfunctorily. Nevertheless, he participated directly in the war effort... In 1777 Mifflin attained the rank of major general but, restive at criticism of his quartermaster duties, he resigned. [More about his military career detailed here.]
...he died at Lancaster [in 1800], aged 56. The Commonwealth of Pennsylvania paid his burial expenses at the local Trinity Lutheran Church.
He was identified as a Quaker by the Library of Congress. A Worthy Company: Brief Lives of the Framers of the United States Constitution by M. E. Bradford was cited as the source stating he was later a Lutheran. (Source: Ian Dorion, "Table of the Religious Affiliations of American Founders", 1997).
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.