Was Barnabas the writer of this gospel? Muslims say, "yes" to this question and call upon the book of Acts for evidence where Barnabas is mentioned. However, our investigation reveals that the Barnabas of Acts and the Barnabas of this gospel are two different people who lived at different times.
According to the New Testament, Barnabas was not present during the ministry of Jesus. He is first mentioned in Acts after the church was already established. He, like other disciples, sold his property to raise money for distribution to the poor (Acts 4). He was a Jew from Cyprus named Joseph whom the apostles called Bar-nabas, which means "son of encouragement" (Acts 4:36).
According to the document in question, however, Barnabas was an apostle of Jesus and known by this name throughout Jesus' ministry. On many occasions Jesus is reported to have called him Barnabas instead of Joseph. This weakens the credibility of this gospel because Barnabas was never called to be an apostle by Jesus, according to all the available lists in the other Gospels.
The Barnabas of this gospel rejects the deity of Christ. Jesus is alleged to have denied being equal with God and to have claimed that he was only a messenger and servant of God. However, in the New Testament in Acts, Barnabas was the one who introduced Paul to the apostles at Jerusalem and encouraged them to trust him. The first thing Paul had preached after his conversion was that Jesus was the Son of God (Acts 9:20) and no doubt Barnabas was well aware of it. If Paul was preaching something unacceptable would Barnabas not have called for him to be silenced rather than fighting for his acceptance as a preacher of God’s word?
This Gospel denounces the teaching of Paul regarding circumcision, the crucifixion, the death and resurrection of Jesus. However, the book of Acts shows that Paul and Barnabas had a joint ministry for one full year. They both preached the same thing concerning Jesus’ resurrection and his deity (Acts 13:33). Here, Barnabas along with Paul can be seen in debate with some who insisted on circumcision. This debate was not between Paul and Barnabas but between the people from Judea on one side and Paul and Barnabas on the other (Acts 15). Thus it is illogical to accept that the Barnabas of the New Testament is responsible for a document that rejects the very teachings he proclaimed and defended.
Muslims quote from Galatians 2:13 and Acts 15:38-40 as proof that there was a disagreement on doctrinal matters between Paul and Barnabas. According to the first reference Barnabas was reprimanded for religious discrimination. Just to avoid offending some Jewish Christians, Barnabas and Peter did not sit at a meal with gentile converts. Paul censured this behaviour. Acts 15:2 shows that Peter and Barnabas later realised their error and accepted Paul’s argument. Muslims may ask, "Was not the root problem an issue of doctrine, namely, justification by faith, not by observing the works of the law, as the rest of Galatians chapter 2 reveals?" The answer is that although it is a doctrinal issue, it does not leave Paul in the wrong. Also, I Corinthians 9:6 which was written after the split, shows that Paul and Barnabas were back on good terms again. We should appreciate that the split reported in Galatians chapter 2 was only momentary.
The case in Acts 15:38-40 relates how Paul did not want to take John Mark with them on their next journey because he remembered how John Mark had abandoned them in Pamphilia in the middle of the first journey (Acts 12:12; 13:13). Paul was concerned that John Mark might do the same again and cause further problems. Later, however, the same Paul commends John Mark in his letters and expresses his need for him in his ministry (Colossians 4:10; II Timothy 4:11).
The evidence reveals that the dispute here was a personal matter and not doctrinal. There is no evidence that Paul and Barnabas had split because of a doctrinal clash. In fact investigation in this case reveals the unity that was between these two men rather than discord.
So why should Barnabas suddenly change his belief? Muslims offer rather fanciful explanations. A favourite theory, for example, is offered by Rahim, who tried his best to propagate this gospel with great zeal in Pakistan and wrote about the life of Jesus as portrayed in this gospel. He says:
Paul was a Roman citizen. He must have learned the language of Rome. He probably spoke Greek as it was the official language of the area in which he was born. The epistles he later wrote to the Christian communities in Greece must have been written in their native language. This meant he could travel in Greece and probably Italy without any language difficulty. Barnabas, on the other hand spoke neither of the two languages. John Mark, who spoke Greek, had accompanied him on the first missionary journey into Greece, to act as his interpreter. If Barnabas was to go there by himself, he would not be able to make himself understood. Thus Paul’s refusal to travel with Mark may have been a round about way of ensuring that Barnabas would refuse to travel with him. 
His statement is riddled with unsupported claims, many of which are easily refuted. The conclusion cannot hope to be valid on such defective premises. It is also inconsistent with the statement in the same book that Barnabas was born in Cyprus. His native tongue would therefore have been Greek and the argument is rendered invalid by his own words. Also, Rahim says that Paul and Barnabas were fellow students under Gamaliel. If such was the case, this would mean that both were educated men who had learned the teaching of the Old Testament and the traditions. The Old Testament was translated into Greek in 250 BC, making it available to the Greek speaking world. If it was so important that there was a need for a translation into Greek three hundred years before Barnabas and Paul, how much greater would be the need of the Greek-speaking Jewish scholars who lived in 30-40 CE? Evidently then, being an educated Jew, Barnabas would not need an interpreter. He would have fully understood what Paul and the other apostles were preaching. Hence there would be no conflict or misunderstanding regarding the deity of Christ, his crucifixion, resurrection and ascension.