While professor he decided to dedicate his life to "making the best biblical scholarship available to the average reader". The eventual result was the Daily Study Bible, a set of commentaries on the New Testament. Despite the name, these commentaries do not set a program of regular study. Rather, they go verse by verse through Barclay's own translation of the New Testament, listing and examining every possible interpretation known to Barclay and providing all the background information he considered possibly relevant, all in layman's terms.
While this detailed approach is not to everyone's liking, the 17 volumes of the set were all instant best-sellers and continue to be so to this day. A companion set giving a similar treatment to the Old Testament was endorsed but not written by Barclay.
Barclay wrote many other popular books, mostly in the same accessible but scholarly style. In The Mind of Jesus (1960) he states that his aim was "to make the figure of Jesus more vividly alive, so that we may know him better and love him more".
He has been accused of being a liberal theologian, of denying both the inerrancy of scripture and the divinity of Christ. His books continue to sell well.