The evidence that these two books are forgeries is pretty strong, though not the strongest of all the NT books. Compared with Paul's other letters, Colossians and Ephesians use longer and more intricate sentences (e.g. Col. 1:3-8 & Eph. 1:3-14 are single sentences in the Greek), have more theologically developed views (e.g. Col. 1:15-20), use different vocabulary and use common terms differently. But Ehrman focuses primarily on one big difference in viewpoint.
"...buried with Him in baptism, in which you also were raised with Him through faith in the working of God, who raised Him from the dead."
"...even when we were dead in trespasses, [God] made us alive together with Christ (by grace you have been saved), and raised us up together, and made us sit together in the heavenly places in Christ Jesus..."These two books explicitly state that they have already been raised. The difference may seem minor, but to Paul such a claim would have had enormous theological ramifications. The writers of Colossians and Ephesians, on the other hand, apparently didn't quite catch the distinction.
Due to similarities in theme and language, virtually all scholars agree that these three books (the pastoral epistles) have the same author—but they also agree that the author is not Paul, as he claims to be. First Ehrman explains the argument from vocabulary:
"There are 848 different Greek words used in these letters, of which 306 do not occur anywhere else in the letters allegedly written by Paul in the New Testament. ...Something like two thirds of these non-Pauline words are words used by Christian writers of the second century. That is to say, the vocabulary of these letters appears to be more developed, more characteristic of Christianity as it developed in later times."Next he points out conflicts in the use of important terms. For example, in all of Paul's other writings, the word "faith" roughly means "trust in God." But in the pastoral epistles, it instead means "the beliefs that comprise Christianity" (e.g. Titus 1:13-14).