Tells volumes that a question like that even could even be asked.
Who knows how many of the previous Presidents were not really Christians but just pretended to be? It's always politically safer to have people think you are a Christian (except perhaps in a few Congressional districts where it may be equally helpful to be Jewish or Buddhist).
Whether Washington was a believer is disputed. I read somewhere that in all of his surviving writings he never once mentions Jesus by name. We do know that in one year before the Revolution he attended church 15 times, but went foxhunting 49 times (the golf of 18th-century Virginia?). Jefferson was a deist. Lincoln seems to have been one too.