Collections of related texts such as letters of the Apostle Paul (a major collection of which must have been made already by the early 2nd century) and the Canonical Gospels of Matthew, Mark, Luke, and John (asserted by Irenaeus of Lyon in the late-2nd century as the Four Gospels) gradually were joined to other collections and single works in different combinations to form various Christian canons of Scripture. Over time, some disputed books, such as the Book of Revelation and the Minor Catholic (General) Epistles were introduced into canons in which they were originally absent. Other works earlier held to be Scripture, such as 1 Clement, the Shepherd of Hermas, and the Diatessaron, were excluded from the New Testament. The Old Testament canon is not completely uniform among all major Christian groups including Roman Catholics, Protestants, the Greek Orthodox Church, the Slavic Orthodox Churches, and the Armenian Orthodox Church. However, the twenty-seven-book canon of the New Testament, at least since Late Antiquity, has been almost universally recognized within Christianity (see Development of the New Testament canon).