His father was a captain in the royalist forces at the time of the taking of Dundee by George Monck in 1651.
Having been parish schoolmaster at Ballingry, Fife, and then Tippermuir, Perthshire, he entered on trials before Perth presbytery on 17 December 1673, and gained testimonial for license on 3 June 1674.
In 1688 Ross, being then primate, nominated him to a divinity chair at St Andrews, but the completion of the appointment was prevented by the abdication of James II.
Driven from Glasgow by the Cameronian outbreak, Sage made his way to Edinburgh, and took up his pen in the cause of the extruded clergy.
After a few months he became domestic chaplain, at Falkirk, to Anne, dowager countess of Callendar, and subsequently to Sir John Stewart of Grandtully, Perthshire.
John Gillan in his Life of Sage gives a long Latin inscription intended for his tomb.