If the Bible was a whitewashed version of the truth, then the apostles would be presented in a much more positive light than they are. This is evidence for the believability of the gospels–that the apostles come across in a realistic way. They often misunderstand Jesus and they often act in very human says. Sometimes they argue over who is the best. Sometimes they totally misunderstand Jesus. If the gospels were a fictionalized account of what happened in the ministry of Jesus, then the apostles would be presented as always agreeing with Jesus, always agreeing with one another and as practically perfect people. I assume the person you are hearing this charge against the Bible from is Muslim. In the Qur’an, the prophets are presented as nearly perfect and sinless. This is definitely not the case with the Old Testament! Which is the more believable account; one written by the Jews themselves which presents them in often unflattering terms, or one written more than a thousand years later by Arabs who did not even know the prophets directly who make the prophets appear as almost flawless people? That is an easy question. The same can be said about the New Testament. The Bible has a very realistic depiction of the major characters in the stories presented. Far from being evidence that the events presented in the gospels are untrue, I believe that this is strong evidence that they are true.
It is true that one of the things that the apostles did not “get” was when Jesus told them that he would be raised from the dead on the third day. He compared himself to Jonah, who was saved from the fish on the third day (Matthew 12:28-32). He told them that the temple would be destroyed and that he would raise it up on the third day (John 2:19-23). In Matthew 17:23 Jesus says specifically that he would be raised from the dead on the third day. All of this was a hint to them that he would be resurrected. The fact that they did not get it is not evidence that it did not happen. In what way of thinking is the fact that people are wrong about something evidence that it is not true? This is not even a logical argument. It certainly is not a contradiction. If I believed that the 1967 Arab/Israeli war did not happen, is that evidence that it did not happen? Or is that evidence of my ignorance? In the case of the gospels, the fact that Jesus raised from the dead, but that the apostles struggled to believe it at first, shows their skepticism, not their being gullible. It shows that they were not inclined to believe in the resurrection, which make the fact that they later believed in it when they saw Jesus face to face a number of times even more believable. This is strong support that the stories are true, not that they are untrue.