So what? So far if we look back really, they've been very bad at everything they attempted. Most of them were just fucking retards and denounced by their community.
Now if you want and go celebrate innocent regular people being massacred in their place of worship, you invite the same in return. And you'll get some of the clever ones into this too. Not a 70 IQ radicalized by Discord faggots, but someone who actually feels justified.
If it was your church being attacked and Muslims were celebrating, I don't think you would take it lying down.
I am not even religious, so I really don't care from this perspective. I just don't think it's very smart to celebrate bottom of the barrel rampage that could very well happen to you next.