I've seen those arguements before, a few of them were posted only a few months ago here and were dealt with, a Muslim actually apologised for posting the "The Gospel of Barnabas" issue after it had been made clear to him that the book was most probably written from the 8th to the middle of the 15th century A.D because certain themes are from that time and there are many many mistakes made, no one from the time of Jesus would make the mistakes that the autor in question made. It is thought that a muslim actually wrote the book so as to agree with the Qur'an, here is what an Islamic scholar said,
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)"
Barnabas was never an apostle of Jesus.
The Gospel of Barnabas is known only in Italian, and no ancient writer ever referred to it. It mentions things that were not used until centuries later. Furthermore, other gospel forgeries written in Arabic were also found in Granada. They were discovered after 1588, and the forgers were Moors. Though one Muslim writer, Ata ur-Raham, has confused this with another writing called the Letter/Epistle of Barnabas, there is no similarity except for the name.
The book contradicts the Bible and the Qur'an, i.e
Jesus is not the Messiah. ch.83 p.181 ch.97 p.223 ch.42 p.97
(Jesus is called the Messiah/Christ in Suras 5:75; 5:17 (2x), 3:45; 4:157, 171, 172, 9:30)
The Messiah is Mohammed. ch.97 p.225-227
A prophet’s words are only to the people they were sent to. ch.43 p.101 (Sura 4:150-151 says not to separate between messengers)
Ishmael was the ancestor of the Messiah. ch.190 p.425; ch.191 p.407; ch.208 p.459; ch.43 p.103
God created all things for the Messiah. ch.191 p.427
God created everything for Mohammed ch.39 p.91 "[Mohammed] shall be my messenger, for whom I have created all things; who shall give light to the world when he shall come; whose soul was set in a celestial splendour sixty thousand years before I made anything."
"The messenger of God [Mohammed] shall answer: ‘O Lord, I remember that when though didst create me, thou saidst that thou hadst willed to make for love of me the world and paradise, and angels and men, that they might glorify thee by me thy servant."ch.55 p.131. Also ch.56. p.133
Without faith in Mohammed, none will be saved. (Most Muslims do not believe you should have saving faith in Mohammed.) ch.192 p.429
Differences with the Bible
Jesus is a voice in the wilderness. ch.42 p.97
Angels "rolled" the soldiers away for Jesus. ch.153 p.355
Mohammed is coming. ch.44 p.105
Differences with the Qur'an
Faithful Muslims who do not have works will be in Hell for 70,000 years. ch.137 p.319
Mohammed will go to Hell and be terrified as he beholds the punishment of others ch.135 p.315
Mary gave birth to Jesus without pain ch.3 p.9
Unlawful to hate anything except sin. ch.86 p.199
God is a father. ch.133 p.307
God is our Father. (-no sons, though) ch.17 p.31,33
There shall be no envy in Heaven ch.177 p.401
General Errors
Jesus went to the Sea of Galilee, and having embarked in a ship sailed to his city of Nazareth. ch.20 p.41 (Nazareth is inland.)
Romans said the idols were almighty. ch.152 p.353
Canaanites despised Pharisees. ch.144 p.335
Pharisees in Jesus’ time very strange in their ascetism. ch.145 p.337-339
Funny things about Pharisees. ch.143 p.343-345
Rome has 28,000 gods. ch.152 p.353
Roman Senate decreed that none should call Jesus God of Son of God. ch.98 p.227, or speak of Jesus Ch.157 p.367
Roman Senate’s decree. ch.210 p.461
People who preached penitence were called Nazarenes (after Jesus). ch.194 p.433
Archangels were Michael, Raphael, Uriel. ch.209 p.461 & ch.215 p.471 & ch.220 p.485
Judas smiled when the disciples mistook him for Jesus. ch.216 p.471
Israel say Jesus was God or the Son of God. ch.138 p.321
Barnabas was one of the disciples of Jesus. ch.83 p.191 & ch.88 p.205 & ch.19 p.39 & ch.72 p.167
There was a great famine in Israel in Jesus’ time. ch.138 p.321
God gave Jesus bad consequences because others called Jesus God. ch.112 p.257
The mountains (plural) of Samaria. ch.81 p.189
Historical Anachronisms
Historical problems mention European things of the Middle Ages, which are out of place in Jesus' time.
Coins in chapter 54 (golden denarius divided into sixty minuti) were Spanish.?
Abraham’s father claimed there were an infinite number of gods. (The Sumerians did not have the concept of infinity) ch.26 p.57
"Whereupon, as the food was going down [Adam’s throat], he remembered the words of God; wherefore, wishing to stop the food, he put his hand into this throat, where every man has the mark." ("Adam’s apple" was first a Medieval European phrase) ch.40 p.93
Pilate was governor of Judea when Jesus was born. ch.3 p.7
Jews taught to fast, do alms, make prayer, and go on pilgrimage. ch.89 p.207
Jubilee is now every 100 years. ch.83 p.191-193
Kings’ barons. ch.131 p.301
You desire horses like knights. ch.69 p.159
The burden of the republic. ch.69 p.161
Courtiers. ch.133 p.307
After the nightly prayer. ch.131 p.299
Pinnacle where the scribes used to preach. ch.127 p.291 & ch.129 p.297 & ch.12 p.19
Prodigal son, new [leg] hose. ch.147 p.241
God is not composite. ch.161 p.377
Man is composite ch.168 p.389
Lazarus and his two sisters were proprietors in other towns of Magdala and Bethany, just like in the Middle Ages! ch.194 p.433
Jesus (really Judas) was dressed as a juggler. ch.217 p.475
Pine-cones. ch.113 p.259
Fistula. (An opening in the body for the purpose of drainage) ch.120 p.275
Jesus could not read at age 12. ch.9 p.15
Do penance. ch.121 p.277
Jesus made prayer in union with the messenger of God and heard Mohammed’s voice. ch.84 p.195
These "more than a few" errors prove that the book was written during the Middle Ages in Europe.
Clues on Who Wrote This Forgery
An Italian printer named Arrivabene in 1547 published the first Italian translation of the Koran. The writer of the Gospel of Barnabas was not well versed in Biblical history nor in orthodox Muslim theology, but apparently he (or she) was very knowledgeable of European Medieval customs. The Italian of the Gospel of Barnabas had evidence of both Venetian and Tuscan dialects. Latin spellings showing Latin Vulgate influence. There are also influences from Dante's works.
There were Arabic notes in the margin. However, as David Sox (p.51) mentions, they were not written by an orthodox Muslim. The Raggs conjecture that since the dark green, oriental-type binding is very similar to the binding of a Turkish document of 1575 in the Venetian archives, the binding and marginal notes could both have been done in Constantinople.
First Suspect: Fra Marino was the father inquisitor of Venice from 1542 to 1550, and perhaps his motive was revenge (Sox p.68). Felice Peretti (the future Pope Sixtus V) was a severe, devoted inquisitor of Venice who made many enemies. In the sixteenth century there were 843 trials for Protestantism and Anabaptism, 65 for blasphemous speech and 148 for sorcery in Venice alone. (Sox p.57) In the 1530’s Venice was criticized for its tolerance. An Augustinian friar was punished for teaching heresy at the Church of St. Barnabas in Venice. (Sox p.59) A handwriting analysis of Fra Marino's handwriting and the Gospel of Barnabas show they could have come from the same person according to Sox p.70.
Second Suspect: Anselmo Turmeda (who later became Abd-Allah ibn Abd Allah) from Majorca, Spain, studied in Bologna for ten years. In his biography, written 1383-1390, he claimed to be a priest before his conversion to Islam. His teacher at Bologna was a crypto-Muslim. De Epalza (p.63-64) says he was a converted Franciscan who took revenge on Christianity after his conversion to Islam. The mention of Spanish coins in the Gospel of Barnabas supports this theory.
Other Suspects: Other Gospel forgeries, these at least written by Moors in Arabic, are from Grenada. None was known prior to 1588 though.
Imagine you were a Muslim who was told that someone found a lost "book" from God. Among other things, this "Sura" mentioned that Mohammed sailed on a boat to Mecca, and this Sura contradicted the teaching of the Bible and contradicted the Qur'an on ten points. The oldest manuscript of the alleged Sura was written in Italian, which is not only not a Mideastern language, but Italian did not exist in the time of Mohammed. Finally, this supposed Sura had some historical customs, which did not occur until 1,000 years later in Europe.
I think it is safe to say a Muslims would probably have a few questions, to say the least. Before you embrace this medieval forgery as an authentic work that shows the "real" teachings of Jesus, remember that this work contradicts the Qur'an, too.
In regard to the rest, there is no point in quoting from the Qur'an because it is of no benefit to Christians.