That's completely absurd. Abraham Lincoln's belief in God was very evident not only in what he said to the masses but in his personal behavior, especially during his presidency.
For at least the majority of his adult life he would have been more of a Deist than a Protestant. He firmly held on to the idea of a moral God who shaped and gave order to events. His views on Providence and predestination reflect his Calvinist upbringing. It's hard to say exactly what else he believed and when; he was a very private person, and from what hints we do have, his positions on doctrine seem to have been fluid.