In 1673, he published “_A Catechism and Confession of Faith_,” which is an exposition of the doctrines and principles of the Quakers, supported by an appeal to Scripture testimony. His next publication is the “_Theses Theologicæ_,” which were addressed “to the clergy of what sort soever,” and contain fifteen propositions, on which he gives his sentiments, and explains them in conformity to the principles of his sect. He vindicated his _Theses_ from the strictures of Nicolas Arnold, professor in the university of Franquer in Friesland, by a Latin treatise printed at Amsterdam, in 1675. In the same year, he published an account of a disputation between the students of divinity of the university of Aberdeen and the Quakers, in which he bore a conspicuous part; but, like most religious controversies, it terminated without satisfaction to either party. The students also published an account of this conference, in a pamphlet entitled, “_Quakerism canvassed_,” which occasioned a reply, entitled, “_Quakerism confirmed_,” in two parts, both printed in 1676. Previously to this time, he generally resided at Ury with his father; but in this year he went to London, and thence to Holland, accompanied by William Penn, the celebrated and amiable proprietor of the province of Pennsylvania. These religious men travelled in Holland and Germany, visiting their friends, and disseminating their doctrines. They waited upon Elizabeth, princess-palatine of the Rhine, at her residence at Herwerden, and were kindly received. She seems to have adopted their doctrines, for she openly patronised the Quakers; but her friendship for Barclay was sincere and unfeigned, and lasted during life. She frequently wrote to him with her own hand; and always promoted his views at the court of England, as far as her influence could be of service to him or his friends. When he returned to London, he learned that his father and other Quakers were imprisoned in Aberdeen, for holding meetings in that city. He therefore presented a memorial in their behalf to Charles II. which was delivered by himself into the king’s own hand, who caused his secretary of state, the Duke of Lauderdale, to underwrite upon it a favourable reference to the council of Scotland, which had the desired effect, as they soon after obtained their liberty.