The vast majority of Muslim academics and scholars of comparative religion have deemed the so-called Gospel of Barnabas as pseudepigraphical, meaning that it is a forgery, probably a pious fraud, that was written for polemical reasons and dubiously attributed to the apostle Barnabas, the traveling companion of Paul according to the Book of Acts in the New Testament.
From the standpoint of textual criticism, there is no extant manuscript of the gospel that predates the sixteen century of the common era (CE); while the oldest extant manuscript dating from this period was written in Italian. The entire Italian manuscript was translated into English by Lonsdale and Laura Ragg in 1907 along with a lengthy introduction.
Although much of what is written in the gospel agrees with the Islamic theology/Christology, such as the presence of many clear predictions of the Prophet Muhammad (peace be upon him) as well as several denials of Christ’s divine sonship and deity, there are also several problems with the gospel.
Inaccuracies in the Gospel
First of all, the gospel denies the messiahship of Jesus Christ (peace be upon him) thus putting itself into clear conflict with the Qur’an, a definitive proof-text; there are also many anachronisms found within the gospel which suggests a much later date of composition; such as the gospel’s claim that Jesus was born during the rule of the Roman prefect Pontius Pilate or the reference to an annual “forty day fast,” – a apparent reference to Lent, a practice not attested before the fourth century CE.
That being said, we know that there was indeed a Gospel of Barnabas (Evangelium Barnabe) that was in circulation among Christian communities in the fifth and sixth centuries CE which prompted Pope Gelasius I to issue the Decretum Gelasianum listing the gospel as apocryphal (spurious). We do not know what this gospel actually contained, only that it was deemed heresy by the decree of papal authority.
The Gospel and the Epistle of Barnabas
Many Muslim scholars erroneously conflate the Gospel of Barnabas with the Epistle of Barnabas; the latter is found in the oldest complete Greek manuscript of the New Testament on earth called the Codex Sinaiticus (circa 375 CE). The Epistle of Barnabas is completely different than the so-called Gospel of Barnabas of the sixteen century CE Italian manuscript. Therefore, serious academic inquiry into the gospel reveals that Muslims should approach it with extreme caution as it certainly appears to be of very dubious origins.
And Allah knows best.