The IRS is the law enforcement arm of the Federal Reserve System in the U.S., lying to them is dangerous. The fact that The Satanic Temple
declared themselves to be a Christian organization cannot be satire as the co-founder, Mesner, claims, because the IRS is not an organization that appreciates jokes, nor do I believe that it is for the expedience of their political activism, which he has given as an alternative explanation, since that, too, appears to be a cover and a ruse to garner donations. So, there is
every reason to take the Satanic Temple at face value when they state on an official document, signed under penalty of perjury, that they are Christians.
They appear to be an offshoot of a religion whose members are believers in God, whom they also call Jehovah and Jesus, whose dark side they call both Satan and Lucifer. This religion is an offshoot of Scientology, which is known as the Process Church of the Final Judgment. That organization is dissolved and now there is a new version of it known simply as The Process, to which Mesner quite evidently has a very close connection.
You can see for yourself
the original web pages of the Satanic Temple before they decided that it would be more expedient for them to pose as atheistic Satanists