Colossians & Ephesians
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.
In most of his epistles, Paul is adamant that Christians who've been baptized have "died with Christ" and been set free from sin, but have not yet been "raised with Christ." He emphasizes that only later will they be raised with him (e.g. Romans 6; 1 Cor. 15). In contrast, here are Colossians 2:12 and Ephesians 2:5-6:
"...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.
1 Timothy, 2 Timothy & Titus
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).
Most importantly, though, are the starkly contrasting portrayals of the early Christian church. In 1 and 2 Corinthians, Paul's churches are portrayed as eschewing hierarchical leadership in favor of letting the Holy Spirit work among all the members and giving each a "spiritual gift" such as teaching or prophecy (see 1 Cor. 12-14). When Paul writes to rebuke the sinful ways of the Corinthian church, he addresses all the members—he can't address the church bishops because there weren't any. While this setup was chaotic, it was okay with Paul, because he believed that Jesus' return was imminent: they just needed to support one another until the end came.
But it didn't come. And so over the decades, the churches had to organize themselves and appoint a hierarchy of leaders in order to survive. That's what we see in the pastoral epistles: they're all about how to appoint bishops and deacons to various positions, how to deal with false teachers, and so on. Paul would have been long gone by the time the churches had reached this state. The conclusion: someone living in the second century wrote these books under Paul's name so they could influence the churches' policies more easily.
Ehrman goes over several other NT books as well, but this should be enough to illustrate the point. At some point in the future I'll devote a whole post to the authorship of 2 Peter. Although Ehrman doesn't give much space to it, the evidence of forgery there is even more decisive.