Yeah, there are some issues here.

First of all when it comes to Jews, they can be certain who is who because they keep proof of their genealogical tree to prove that they are ...Jews.

Muslims also consider themselves as a minority and they are united.

When it comes to us, Christians, how you can have a "proof" of the christian identidy?

Because he/she is baptized? Or they have to show you proof that they are regulars to a parish?

And what about baptized atheists or agnostics?

Or trade only local?

And if you want something you can't find local?

Ancient Christians were united under Rome's tortures and persecution.

They had some "signs" or "code" lime the Greek acronym "ΙΧΘΥΣ" that only them knee: the original meaning is fish but also the first letters of "Ιησούς (Jesus)", "Χριστός (Christ)", "Θεού (God's)", "Υιός (Son)", "Σωτήρ (Saviour)".

So how this can be done today?