I would say that becoming a Citizen of the Infernal Empire (a member of the Church of Satan
) is the true
By doing so, you formally ackowledge yourself as a Satanist.
The baptism is an optional ceremony that Active Members and members of the Priesthood of Mendes may or may not perform in private. Because becoming a member is the first step, an actual baptism would be something like reaffirming wedding vows. It would no doubt be a powerful ritual if it was something that you truly wanted, but unless you already know some Satanists willing to perform such a ritual with you--you are out of luck.
Don't put the cart before the horse.