Wittgenstein suggests the most blatant mistakes in religion could have been on purpose, what could they mean?

Well, since you are speaking theoretically, depending of course on the version of the bible you adhere to, on the 4th day, do SEPARATED the light from the darkness, not that he created the sun. It is reasoned that he "created the moon and the sun on this day" from the context, but that could simply imply that he gave the light a shape and form, as opposed to an abundance. You ca go out at dusk when neither the sun nor the moon is visible and still be in light, In some ways it might be as if to say, on the first day God made the day in parts, both lit by some force, but it was not until the 4th day that these separate entities of light, the moon of the night, the sun of the day, were given true shape and form and function. This occurred on the fourth day.