While Samuel Langhorne Clemens (aka Mark Twain) wasn't an atheist, he certainly wasn't a Christian, either.
Closer to say he was agnostic.
https://www.thoughtco.com/religion-quotes-by-mark-twain-283266
"Man is a Religious Animal. He is the only Religious Animal. He is the only animal that has the True Religion -- several of them.
He is the only animal that loves his neighbor as himself and cuts his throat if his theology isn't straight."
"So much blood has been shed by the Church because of an omission from the Gospel:
'Ye shall be indifferent as to what your neighbor's religion is.'
Not merely tolerant of it, but indifferent to it. Divinity is claimed for many religions; but no religion is great enough or divine enough to add that new law to its code."
"The higher animals have no religion. And we are told that they are going to be left out in the Hereafter."
"The Christian's Bible is a drug store. Its contents remain the same, but the medical practice changes."
Samuel Langhorne Clemens aka Mark Twain.
https://www.thoughtco.com/religion-quotes-by-mark-twain-2832666
"In religion and politics people's beliefs and convictions are in almost every case gotten at second-hand, and without examination."
"A religion that comes of thought, and study, and deliberate conviction, sticks best."
"It ain't those parts of the Bible that I can't understand that bother me, it is the parts that I do understand."
"No God and no religion can survive ridicule. No political church, no nobility, no royalty or other fraud, can face ridicule in a fair field, and live."
Samuel Langhorne Clemens aka Mark Twain
Personally, I believe he was only off in the failure to mention the matter of organization.
Remove the organization, you remove religions harmful effects on society as a whole.
If you really want to assign satanism to Samuel Clemens, you really don't understand satanism at all.
For he mentions nothing about Satan in ANYTHING! And really isn't against ANY God, or goddesses or deities.
Nor is he supporting any form of atheism.
He only has brought up the many faults of ANY religion.
You are guilty of the very thing Samuel Clemens has warned about religion, that includes many other things like fraud.
"No God and no religion can survive ridicule.
No political church, no nobility, no royalty or other fraud, can face ridicule in a fair field, and live."
