Now I think that this says something very interesting about how most liberal/mainline Protestant perceive religion. According to them, lower forms of religion need real things to have happened in the real world. Even if these literal religionists can prove their case to a certain degree using historical evidence, the fact that they even try (and believe me I've heard this over and over again!) shows immaturity. Grown up religion, according to these people, means a "spiritual" and "ethical" religion. In other words, if the religion is about telling us general ethical truths and not particular promises God has made and fulfilled (Abrahamic covenant, exodus, crucifixion/resurrection, etc- all which would demand literal historical truth on the part of the Bible.), then who needs anything to have really happened? The ethical principles are still good even if their was no Abraham and no conquest by Joshua. Also, these presupposition would also suggest that God is spiritual and therefore doesn't do real stuff in the real world. In other words, Incarnation is out- or if they accept it, then a series of contradictory principles are accepted along side it.
In the end, "literal religion" is not childish religion. Rather it is evangelical and incarnational religion. It's about God fulfilling real promises, in the real world, through a real imparting of himself in Israel's history and in Christ. The other kind isn't more mature, it's simply moralizing and spiritualizing- the religion of our first parents, as Luther would say.