"Satan himself is converted after the modern order of things".

"Satan himself is converted after the modern order of things". - GC ch 36

His religion is modern Spiritualism. LOA. Reincarnation.

Satan himself is an angel who was never incarnated as a human. He has never looked at himself as a human. In fact He derided Christ for sacrificing himself, being incarnated as a human. He despise humanity. Notice in one of his messages as the counterfeit Christ, where he states that humans can never reach the height and level as himself. He regard himself still as unfallen.

As for reincarnation: as an angel who was never incarnated, which in his mind is to become fallen, he would see it as perfectly reasonable that he would never be incarnated or reincarnated, which is his religion.

So, I believe it is reasonable to think that the fact that he managed to actually have the power to murder Christ on the cross, as a sign of his power being greater than that of Christ. Both the fact that Christ was incarnated and that he managed to murder him, he probably regard as a sign that he himself is superior.

However, he must also understand that there is a strong possibility that he is wrong. The fight has continued since.

Notice, he takes pride in the flesh, in the external, in the LETTER.