Actually Josephus, Tacitus and Philo of Alexandria mentions him in their writings. None of these writers were Christian. As far as I know, no one has doubted the existence of Pontius Pilate.
> Actually Josephus, Tacitus and Philo of Alexandria mentions him in their writings. <
Josephus and Tacitus were not adult contemporaries of Pilate. Their writings were based on what other people told them. Only Philo was an adult contemporaries of Pilate. And Philo lived in Egypt, not Judaea. He never saw Pilate. That single source (Philo) was not enough for many scholars.
> no one has doubted the existence of Pontius Pilate <
Many secular historians left his existence as a question mark. Remember, those folks were working hard to deny the truth of the Gospels. There was no hard proof that Pilate existed until the Pilate Stone was discovered in 1961.