 So it was that in 1612, aged forty-eight, Forbes received his ordination into the ministry and became minister of Keith, in the diocese of Moray.Forbes left a number of theological writings from this period, including "An Exquisite Commentarie upon the Revelation of Saint John" (1613) and "Short Discovery of the Adversarie" (1614).In the former, he was vehemently anti-Catholicism and argued that the Catholic Church had become corrupted by the greed of bishops even before the reign of the Emperor Constantine the Great, becoming irrecoverable during the papacy of Pope Boniface VIII.