The Gospel of Barnabas is a long Italian work written in the fourteenth century AD by a person who had converted to Islam. The writer may have wanted to take advantage of the mysterious mention of such a gospel in the Gelasian Decree, which was written no later than the sixth century AD.
So far, there is no evidence that a genuine or ancient gospel written by Barnabas ever existed. Because of this, many scholars believe the Gospel of Barnabas is not authentic but a much later imitation.