The author makes much use of non-New Testament sources, especially Apocrypha, early church fathers, etc. Much is also speculative, and pretty thin on the more obscure Disciples. And Barclay has some heterodox views (not seen in this book, though). Still, there is much that is good and interesting here, and the book is well-written.
In explaining the phrase “Son of Man,” Barclay (22) cites the Book of Enoch (“a pre-existent, celestial, supra mundane figure waiting in the heavenly places to be unleashed to fulfill God’s purposes in the world and among men”).
I was struck by how fantastic some of the Apocrypha, early church legends, and so forth are. On the one hand, it is worrisome that such things were being written at the same time as the New Testament canon. But, on the other hand, it just goes to show that the church did decide to look at what was worthy of the canon and what was not. The fact that the Catholics include the Apocrypha in their Bible and the Protestants don’t makes sense, by the way: No sense including dubious material if non-experts are expected to read it.