Brought up a Presbyterian faith, he studied and adopted Unitarian doctrines, at a high social cost.
Writing to Joseph Priestley in 1781 he stated that, so great was his unpopularity, that he did not suppose any Scottish minister would baptise his children.
By Priestley's mediation, Caleb Rotheram of Kendal visited Montrose to perform the rite.
He there delivered sermons which he later published under the title of ‘Dissertations on the Unity of God,’ and issued proposals for the publication of a series of lectures on the Revelation of St. John, but the project met with no encouragement.
After periods at Winchester, Virginia and Northumberland, Pennsylvania, where he delivered an address at Dr. Priestley's funeral on 9 February 1804, he settled in Philadelphia, where for some time he was the minister of a small Unitarian congregation.