The following is from the FAQ's section in Joy of Satanas segment about doing the Dedication Ritual to Satan.
Question: I am underage, living in a Christian home, and my parents force me to go to church and participate in Christian sacraments. Can I still do the dedication? Will Satan be angry with me?
Answer: Yes, you can still perform the ritual. Satan understands. As long as you are loyal to him in your heart, he will not be angry with you. Those of you under 18 are not free. There is no need to reveal your allegiance to Satan to anyone. What is in your heart is more important than anything else. Satan advises us in the Al-Jilwah not to reveal our religion to outsiders if it will cause us harm in any way; this is especially important for teens. Just renounce the Christian "God" in your mind if you are forced to participate in any of his garbage. Satan understands it can be downright dangerous and, in some cases, even life-threatening for teens living in Christian homes to reveal their allegiance to him.
So, if you end up getting baptised, just say in your mind, " I do not follow the God of christians. I am a disciple of Satan." say this in your mind while being baptised and that should negate or make the christian baptism null and void. no stigma on your soul.
Hail Satanas!!!