Scottish Presbyterian clergyman denounced by his church for opposing the concept of a national religious establishment. He was founder of the Glasites (Sandemanians (Sandemanian), q.v.).

Glas became minister of Tealing Church, Dundee, Angus, in 1719. Some of his parishioners led him to question the scriptural basis for the national Presbyterian Church to which he had been loyal, and he soon concluded that the New Testament provided no evidence to support foundation of such a church. He viewed the kingdom of Christ as an essentially spiritual one and considered national covenants and civil magistrates as having no valid ecclesiastical function. He particularly opposed the maintenance of the church of Christ by political and secular weapons. These arguments are expounded in his major work, The Testimony of the King of Martyrs (1727). After organizing a society of independent Presbyterians within his own church and neighbouring parishes, Glas was summoned before his presbytery in 1726, suspended in 1728, and deposed in 1730. Known as the Glasite Church, his society soon moved to Dundee.

In 1733 Glas began working in Perth, where he was joined by his son-in-law Robert Sandeman (Sandeman, Robert) (q.v.). In 1738 Glas returned to Dundee and the following year was restored to his function as minister, though without official status as a minister of the established Church of Scotland. Most of his works were collected in a five-volume edition (1782–83). Through Sandeman his teachings survived to form the basis of sects founded in England and the United States, though most Glasites eventually were absorbed by other denominations; the last of the Sandemanian churches in America ceased to exist in 1890.