Arguing from textual analysis, he advanced a proto-gospel hypothesis, a variant and modification of the contemporary claim by Johann Gottfried Eichhorn.

His Dissertation (1801) deduced that there had been an original Aramaean gospel-narrative which had been translated into Greek, and had been circulated in copies into which additional information was afterwards added or interpolated.

[1] In 1805 he began to preach against Calvinism in a series of sermons "in which he denounced the doctrines of justification by faith without works, and of the impossibility of falling from grace, as giving a license to immoral living",[4] which brought him into conflict with the Evangelicals, such as Charles Simeon and Isaac Milner.

[4] In 1807 he resigned his fellowship at St John's on being elected as the Lady Margaret's Professor of Divinity at Cambridge and began presenting lectures there on Higher Criticism.

[1] As a bishop, Marsh was controversial for preaching against the Evangelicals and for refusing to license clergy with Calvinist beliefs (for which he incurred the ire of Sydney Smith).