Because religious vocabulary, etiquette, and clothing are learned from decades of involvement usually as a small child.

It’s kind of like saying you’re going to pretend to be a different class, but more particular.

> I have pretended to be multiple kinds of Christian with success

I don’t know what context you mean. But I guarantee others can tell you aren’t a member of the group even if Christianity aims to be open in inviting new people.

Another common situation is they recognize you have some connection but can tell you aren’t an active participant.