Why do Christians say that The Gospel of Barnabas was (1) made between A.D. 1300-1350 (2) in Medieval Europe (3) possiably by Muslims and is thus (4) a fake? (5) It Also Contradicts the Quran.
1) The Gospel of Barnabas was made between A.D. 1300-1350.
Chapters 82-83 "Years of the Jubilee...every 100 years." The Year of Jubilee is every 50th year. In the year A.D. 1300 Pope Boniface VIII instituted the Jubilee as a centenary (100 year) event. Pope Clement VI reversed Boniface's decision and celebrated the next Jubilee in 1350. This was the only time that the Year of Jubilee was intended as a centenary occasion.
Chapter 3 "The birth of Christ is described as having been painless." This belief was not part of the Church before Thomas Aquinas (who died in 1278).
The Italian poet Dante lived (1265-1321). It is interesting to notice a number of similarites between Dante's works and the G.o.B. (Chapter 106 corresponds with Cantos XXVIII  and III  of the "Inferno"). Barnabas agrees with Dante and contradicts the Quran at the same time. The Quran (Sura 2:29) says that there are 7 heavens, while Barnabas says that there are 9 heavens (Chapter 178).
Not before the Fourth Century A.D. was the title "Virgin" given to Mary (See GoB, Chapter 209, 218).
In Chapters 91-92 "Jesus and His disciples kept the 40 days of Lent". During the reign of Charlemagne in the 900's, four more days were added (to the 36), to create the current 40 day celebration of Lent. Also Jesus and His disciples are said to have gone for the 40-day fast to Mount Sinai (GoB, Ch. 92) which is about 450 km away.
Chapters 40 and 41 Eve is said to have eaten an "apple" as the forbidden fruit. We are know that Eve ate an unspecified fruit. The belief that this was an apple dates from a much later date. Another forbidden food was corn (or wheat) in Chapter 39.
2) The Gospel of Barnabas was made in Medieval Europe.
In Chapters 137 and 220 of the G.o.B. it mentions four archangels or favorite angels, which is a tradition of the church that dated from the early Medieval period.
Chapter 54 of an Italian text of the GoB mentions a denarius, which is made up of 60 minuti. These gold coins were used only in Spain under Khalif Abdul Malik (in 685 A.D.).
Chapter 152 "Soldiers rolled out of the temple like casks of wood." Wooden barrels were invented in Gaul (France) and were not used in the East in New Testament times. They used wineskins (Matthew 9:17).
3) The Gospel of Barnabas was made by a Muslim (who was not orthodox).
We read that Ishmael was offered on the altar by Abraham (Chapter 44). This is clearly an Islamic concept. Genesis 22:9 says it was Isaac on the alter. This is supported and confirmed by the New Testament (Hebrews 11:17). Remember that the Christians and Jews were not friends at this time (Sura 2:113) so a collaboration to change both the New and Old Testaments was impossible.
Chapter 212 "God is said to be the God of Abraham, ISHMAEL and Isaac." It should read, "the God of Abraham, Isaac and Jacob (used in Exodus 3:6, Moses and the burning bush)". This is supported and thus confirmed by the New Testament in Mark 12:26 (have you not read in the book of Moses, in the account of the [burning] bush, how God said to [Moses], 'I am the God of Abraham, the God of Isaac, and the God of Jacob'?).
Most suspicious of course, is any mentioning of the name of Muhammad (In Chapters 44, 54, 112, 97 and 163). Other Islamic thought is also reflected in the G.o.B. Jesus prayed five times a day and all the Muslim prayer times are mentioned. (Drs. J. Slomp, page 128). In Chapter 39 Adam sees the Muslim Kalimah (Creed).
4) The Gospel of Barnabas is a fake Gospel.
In Chapter 97 Muhammad is called the Messiah. Both the Quran and the Bible say that Jesus is the Messiah. In the introduction of the G.o.B. Jesus is called Christ (see also chapter 6). In Chapters 42, 82 and 96 Jesus denies that he is the Messiah. "Christos" (English: Christ) is the Greek word for the Hebrew "Messiah".
In Chapter 20 Nazareth is said to be on the shore of Lake Galilee. Nazareth is a town about 20 km away from the Lake.
In Chapter 151 Jesus embarked on a ship (from Nazareth?) and then he arrived in landlocked Jerusalem.
According to the G.o.B. (Chapter 3) Jesus was born when Pilate was governor, but in fact Pilate only became governor between A.D. 26 and 27.
Chapter 91 "The Romans assembled three armies each numbering 200,000 men at Mizpeh". The entire Roman army at that time numbered only 300,000. (Encyclopedia Britannica). "Even at its greatest size the Roman Army never numbered much more than 500,000" (Eastern Counties Newspapers Group Ltd, found at http://www.ecn.co.uk/now/archive/romans/e_ed_kid_RomanArmy.htm). Could the Romans afford to assemble the entire army in Judea? What about all the barbarian invaders? It is interesting to note that this army was amassed because the Hebrews were split in opinion about the identity (deity) of Jesus.
Chapter 98 "A priest asked the governor to write to the Roman senate [and the governor did] about Jesus teaching that he is not God. The senate had compassion on Israel, and decreed that on pain of death none should call Jesus the Nazarene, prophet of the Jews, either God or son of God. Which decree was posted up in the Temple, engraved upon copper." Did the pagan Roman goverment care that much about the Jews' religious problems dealing with a poor pesant?
Chapter 23 "just as every man takes flesh from the flesh of Adam, so is he bound to observe all that Adam promised with an oath" Are we are bound by Adam's oaths?
5) It Also Contradicts the Quran
(GoB 15) Jesus said "In the name of God give the [guests] the wine [that I made from the water]". Wine is forbidden in Islam (Suras 2:219 and 5:90) it is the "work of the devil".
(GoB 23) Jesus talks about the "obligation of circumcision" and says "Leave fear to him that has not circumcised his foreskin, for he is deprived of paradise". This "obligation" is not talked about in the Quran.
(GoB 69) Jesus says, "blasphemy against the Holy Spirit is without remission [for it is unforgivable]". If this was so, why does the Quran not warn us about this most serious sin? Also why did Jesus say that you could "blaspheme"against the Holy Spirit if the Holy Spirit was not part of The Trinity?
In Chapter 112 Jesus said, "For if men had not called me God, I should
have seen God here as he will be seen in paradise, and should have been
safe not to fear the day of judgment. But God knows that I am innocent,
because never have I harboured thought to be held more than a poor slave.
No, I tell you that if I had not been called God I should have been carried
into paradise when I shall depart from the world, whereas now I shall not
go thither until the judgment. Now you see if I have cause to weep."
The Quran says "No soul can carry the sins of another soul." (35:18, 39:7 and 53:28). The Quran also says that Jesus was raised to God after leaving this world (Sura 4:158).
Chapters 39 and 41of the GoB say "There is only one God and Muhammad is his prophet". Dr. Rashad Khalifa (in "APPENDIX 13" of the "Quran's Translation") says "Millions of Muslims...insist upon mentioning the name of Muhammad besides the name of God [in their creed]. However, the Quran's great criterion in 39:45 stamps such Muslims as disbelieves...". I think he is saying that the creed "There is only one God AND Muhammad is his prophet" is wrong for putting Muhammad's name with God's.