Are you 'in church', 'at church', or 'at the church'?

Carlo: Hi Lovely! How are you? I'm surprised you're answering. I'd just wanted to leave you a 'radio show' message. Aren't you in church? Like you had said in your message?

Amber: Yeah, but we're going to pick up Lindsey

Carlo: Oh, so you're not praying; the mass, or the ceremony isn't on?

Amber: Oh, no.

Carlo: Well, then, I should tell you Love, that 'in church' means you are praying, or the mass, or ceremony, or procedure is in happening.

Amber: Oh.

Carlo: Yeah, and at church could mean the same thing; it's less clear. But at the church, means you are there for another reason, for example, to get your hat (which perhaps you forgot in the pew [the long, bench-like seats]), or to talk to a preacher, or it could mean you are simply near or in the church, but you are not doing anything in relation to the mass; you are not praying. Maybe you are talking to someone.

Amber: Oh. Thank you!

Amber: Oh. Of course!