Sir William Hamilton (1788–1856), in some instances, discussed Cairns's metaphysical opinions at considerable length in the class-room, and Professor Wilson highly eulogised his talents and his attainments in literature, philosophy, and science.

Cairns was associated with Alexander Campbell Fraser, David Masson, and other leading students in organising the Metaphysical Society for weekly philosophical discussions.

In 1843 the movement that culminated in the formation of the Free Church aroused his interest, and an article of his in the 'Secession Magazine' prompted inquiries regarding the writer from Thomas Chalmers.

At the end of 1843, Cairns officiated for a month in an English independent chapel at Hamburg, and he spent the winter and spring of 1843–4 at Berlin, ardently studying the German language, philosophy, and theology.

Returning to Scotland, he was licensed as a preacher on 3 February 1845, and on 6 August of the same year he was ordained minister of Golden Square Church, Berwick-on-Tweed.

Here he became one of the foremost of Scottish preachers—notable for certain quaint but attractive peculiarities of manner, but above all for his force and impressiveness of appeal—and he declined several invitations to important charges, metropolitan and other, and to professorships both in Great Britain and Canada.

Interesting himself in public questions at home, he delivered his first great platform speech at Berwick in 1856, when he successfully combated a proposal favouring the introduction into Scotland of the methods of the continental Sunday.

Next summer he was a fortnight in Paris, in connection with the M'All missions, and on the way formed one of a deputation of Scottish ministers who expressed sympathy with Mr. Gladstone in his attitude on the Bulgarian atrocities.

About the same time he was chairman of a committee of eminent Protestant theologians, European and American, who discussed the possibility of formulating a common creed for the reformed churches.