They had joined the Reformed Presbytery in 1743, but separated from it in 1753, as holders of the doctrine of universal atonement (this split occurred at the death of John Macmillan, and they were without a fixed ministry).
In 1763 the Berwickshire societies sent him as their commissioner to Coleraine, County Londonderry, to consult with a branch of the Irish secession church holding similar doctrines.
A minute expressing concurrence of doctrine was signed at Coleraine by John Hopkins, Samuel Lind, and Purves.
Three candidates delivered trial discourses on 8 June 1769; one of these withdrew from membership: of the remaining two, Purves was selected by lot (27 July), and sent to Glasgow College.
Its theology was high Arian, but its distinctive position was the duty of free inquiry into the scriptures, unbiassed by creed.
In 1792 the worship of the society, in the Barbers' Hall, Edinburgh, was made public, and the name "universalist dissenters" was adopted, and a declaration of opinions was issued.
His congregation was without a minister till the appointment (November 1812) of Thomas Southwood Smith; it later moved to St. Mark's Chapel, Castle Terrace, Edinburgh.