In accordance with the wishes of his father, who had destined all his sons to the ministry of the Scottish church, Patrick devoted himself to theology, and received license as a probationer from the presbytery of Fordoun on 26 March 1817.
He continued to occupy this preferment for nearly forty-one years, and died at his residence at Craigie Linn, near Paisley, on 26 March 1859.
This polemical spirit may be traced in the volume of his sermons entitled The Seven Chartist and Military Discourses libelled by the Marquis of Abercorn and other Heritors of the Abbey Parish.
Brewster advocated the abolition of the slave trade, the repeal of the corn laws, temperance, and a national system of education.
He published three single Sermons, and a vindication, in two parts, of the rights of the poor of Scotland 'against the misrepresentations of the editor of the "Glasgow Post and Reformer."'