Have you considered changing your number so he cannot text you any longer? I am not sure I would want to be in contact with him. Or else, being me, I would start texting back bible verses that would upset him on purpose. Or quotes that were all loving and sweetness and light. I would do it on purpose to make him angry. I am perverse like that. My dad raised me to "Love thine enemy. Drives them NUTS!" No, I am not at all joking. Yes, I did use this on my own difficult child. I can remember when he was telling all the kids at school about how he screamed during his baptism because the water burned his forehead. I overheard him telling a kid this, in gory detail, and came up behind him. I put my arm around him and in a sugary sweet tone told the kid that really, Wiz was the sweetest baby and he coo'd and giggled so much the priest put water on him a second time because he liked it so much. Because THAT is what truly happened.
I ruined all of my son's cool points that day. The cooing and giggling story made it around that school like wildfire! LOL!
If your son is going to purposely try to drive you to fight back, why not have some fun with it? Don't react mad, react smart. What drives him the most crazy? Bible verses? Barney? Blues Clues? Use it. I can remember there were times when my son would do something mean to my daughter that I made him watch a Barney video as punishment. By that time he was old enough to HATE Barney. She loved Barney. WIn for me. If texting is the problem, write down a list of things you could text him that would be responses that would drive him up a wall. Then whenever he texts something mean or angry, respond with one of those. It can be an inspirational quote, a bible verse, a saying from AA, whatever comes to mind. As soon as it isn't fun and he isn't regularly getting his payoff, meaning he isn't getting you mad on a regular basis, he won't text you. Find some fun in making him angry? Oh he will leave you alone even faster. Given his Satanism, start sending him some Christian quotes, tell him you are praying for his soul and that God loves him, and you will drive him NUTS.
NEVER EVER EVER respond again to him when you are angry. If you are angry, just put the phone down. You can just ignore him, or you can send something so sugary sweet that even you cannot stand it. But nothing should SOUND angry. I know it will be hard, but I have faith that you can do it. Prepare responses when you are calm.
It is time to stop the torture and end the nonsense. Make it not fun for him and he will find someone who will give him his payoff. He wants the conflama (conflict plus drama = conflama, our kids are addicted to the adrenaline rush of it) that he gets from getting you angry. Stop letting him get that payoff.
A generic, all purpose response that you can use is "Aren't you sweet?" It is a southern thing. Around here women use it all the time. It can mean what it says or anything between that and you are the exact opposite of sweet. Feel free to channel your inner southern belle and use it. I think tone of voice is what conveys the meaning, and over a text it can have any tone of voice.