There is no concensus. The majority of early church leaders such as Melito, bishop of Sardis, Ireneus, Tertullian, Clement, Eusebius and Papias, bishop of Hierapolis believed that the Revelation was written by John, the son of Zebedee, who wrote the Gospel of John.
On the other hand, Dionysius, Bishop of Alexandria and a disciple of Origen, asserts that it is written by Cerinthus. St. Jerome also expressed doubts of Johannine authorship.
Who is right?
Both camps are right and wrong partly.
Both schools are erroneous in that they both believe the Revelation was written by a single author.
Both are right partly because some parts were written by Apostle John but more than 90% of the Revelation was written by another person, perhaps long after the Gospel of John was written (about 100 AD). It was also the tradition for a disciple to attribute his own writing to his teacher.
The seer states that the Revelation (at least some verses in Chapter 1) was written while he was banished on the island of Patmos, probably toward the end of Domitian’s reign (81 – 96 AD).
Those who believe in Johannine authorship asserts that "logos" or "Word" appears in both the Revelation and the Gospel of John.
John was the only apostle who witnessed the crucifixion of Jesus.
John's
priority was NOT in recording his complex visions or revelations, but
in preserving the teachings of Jesus and the life Jesus lived.
Which part was not written by John?
Most of the Revelation, except some verses in Chapter 1, was written by another, who may also have been called John.
Words
like “life,” “grace,” “light” and
“truth” do not appear in the Apocalypse. Hades or “hell”
does not appear in John’s Gospel, because Jesus did not teach the
doctrine of hell to his apostles, at least according to John.
Revelation
14: 4 states that only 144,000 Jewish men who have not defiled themselves
with women were redeemed. Thus, the latter part of the Revelation was
written by a Jew.
“Dragon”
never appears in the four Gospels, never in Jesus teachings, but appears
numerous times in the Revelation. John never heard the word “dragon”
from Jesus’ teachings.
Jesus
never taught “the wrath of God” throughout
his life. So it does not appear in Apostle John’s vocabulary. But
the writer of the Revelation mentions “wrath of God” six times,
and even “the wrath of Lamb” once. John and the Revelator
are two different personalities. Old Testament authors and even John the
Baptist mention the wrath of God.
John never mentioned hades or hell in his Gospel. Hell is an important
concept in the Revelation.