If you were going to take a stand against religion outside a church...

[Paul the Human]

I can't think of any way to effectively protest a church, but then, I'm not the activist type in the first place. I mean sure, you could point out certain things, like sexual abuse, or that their god is guilty of crimes against humanity, or that science has shown many of their beliefs to be false, etc. But I don't think any of that would be effective against a specific church and/or it's parishioners.

My favorite kind of pro-atheist activism is when people join a group of religious protesters and hold up signs mocking the protesters. That shit is funny!