None of those letters from Abbott and Herndon show that Lincoln is an atheist. Indeed, they are really about the fact that he wasn’t a Christian. Here’s what Abbott says:
> In the first place, Mr. Lincoln’s mind was a purely logical mind; secondly, Mr.
> Lincoln’s was a purely practical mind.
And that proves what?
> His mind, when a boy in Kentucky, showed a certain gloom, an unsocial nature,
> a peculiar abstractness, a bold and daring skepticism.
And that proves what?
> This book was an attack upon the whole grounds of Christianity, and especially
> was it an attack upon the idea that Jesus was the Christ, the true and only-
> begotten son of God, as the Christian world contends.
This book [which Abbott claims once existed] is against Christianity, not against theism.
> From what I know of Mr. Lincoln, and from what I have heard and verily believe,
> I can say, first, that he did not believe in special creation, his idea being that all
> creation was an evolution under law; secondly, that he did not believe that the
> Bible was a special revelation from God, as the Christian world contends;
> thirdly, he did not believe in miracles as understood by Christians; fourthly, he
> believed in universal inspiration and miracles under law; fifthly, he did not
> believe that Jesus was the Christ, the son of God, as the Christian church
> contends; sixthly, he believed that all things, both matter and mind, were
> governed by laws, universal, absolute and eternal.
All this claims only that he was not a Christian.
> If he had been asked the plain question, “Do you know that a God exists?” he
> would have said: “I do not know that a God exists.”
Here Herndon is saying that he never heard Lincoln say that he was an atheist or an agnostic, so he’s going to make up an answer that he thinks that Lincoln would have said.
> The best evidence this side of Lincoln’s own written statement that he was an
> Infidel, if not an Atheist, as claimed by some, is the fact that he never mentions
> the name of Jesus.
Again, all this is only relevant to the question whether he is a Christian.
Why do you think we can believe Abbott and Herndon? They are only two sources among many. Even if they are accurate, all they show is that Lincoln wasn’t a Christian. You’re stepping beyond the evidence when you claim that Lincoln must have been an atheist.