Five New Testament books are traditionally ascribed to John: the Gospel, the three Johannine epistles, and the book of Revelation. Whether one man wrote them is among the oldest disputed questions in Christian scholarship.
The Gospel and the first epistle share a vocabulary and theology so distinctive, light and darkness, truth, abiding, the Logos, that most scholars assign them to a single tradition, often described as a Johannine community or school gathered around a founding figure rather than to the apostle's own hand. Already in antiquity, Papias and later Dionysius of Alexandria distinguished the apostle from another John the Elder or Presbyter at Ephesus, partly to explain the differences.
Revelation is the sharpest problem. Its author names himself simply as John and writes on Patmos, but its Greek is rough and unlike the polished Gospel, its theology and imagery are different, and Dionysius argued on stylistic grounds that it could not be by the same writer, an early and remarkably modern piece of criticism. The mainstream position today separates the evangelist, the author of the letters, and the seer of Revelation into distinct figures, while the traditional churches continue to hold them together in the apostle.