4. When was the Gospel of Barnabas Written?
So far we have seen that the author of the Gospel of Barnabas was not familiar with the language, history or geography of the time of Jesus. He also has several 14th century ideas in his book and the manuscript evidence dates from the 15th century onwards. It therefore is reasonable to conclude that the Gospel of Barnabas was composed in the 14th century A.D. and not in the 1st century by a disciple of Jesus. Is this a reasonable conclusion? It seems so because even some Islamic scholars agree with this dating:
As regards the "Gospel of Barnabas" itself,
there is no question that it is a medieval forgery ... It contains anachronisms which can date only from the Middle Ages and not before, and shows a garbled comprehension of Islamic doctrines, calling the Prophet the "Messiah", which Islam does not claim for him. Besides it farcical notion of sacred history, stylistically it is a mediocre parody of the Gospels, as the writings of Baha Allah are of the Koran. (Cyril Glassé, The Concise Encyclopedia of Islam, San Francisco: Harper & Row, 1989, p. 65, bold added)
http://answering-islam.org.uk/Green/barnabas.htm
This work is written from a highly pro-Islamic viewpoint, not only mentioning Muhammad by name, but including the shahada (chapter 39). It is strongly anti-Pauline and anti-Trinitarian in tone. In this work, Jesus, is described as a prophet and not the son of God, while Paul is called "the deceived". Furthermore, the Gospel of Barnabas mentions in detail that Jesus was not crucified but rather raised alive to heaven—a docetist theme found in the Gospel of Peter that was taken up in the Qu'ran—while Judas Iscariot the traitor was crucified instead. These beliefs in particular that Jesus is a prophet of God, not crucified but raised alive, conform with Muslim beliefs. Other passages however directly contradict the text of the Qur'an; as for instance in the account of the Nativity, where Mary is said to have given birth to Jesus without pain; or as in Jesus's ministry, where he permits the drinking of wine and enjoins monogamy. Narrative themes, and some highly distinctive phraseology, are shared with the Divine Comedy of Dante (Ragg). If (as most students surmise) the Gospel of Barnabas is seen as an attempted synthesis of elements from both Christianity and Islam, then sixteenth and seventeenth century parallels can be suggested in Morisco and anti-Trinitarian writings; but there are no known earlier precursors.
http://www.answers.com/topic/gospel-of-barnabas