Why? Why do they have to be told? Are they sheep? [No, don't answer that!]
And why do they need to do this? I don't give a flying wombat what fictions you believe. I'm glad to say that's your problem, not mine.
And yet most true believers already wear their faith on their sleeves and — very rudely — make sure we're not allowed to forget it. But sure, if they want to wear an emblem, why shouldn't they?** Who is to stop them going around adorned with badges and looking like a bus spotter? Moreover I'm sure the monasteries could find enough spare saints' fingers for them all to have a few poking out of a breast pocket to complete the bus spotter look. The same applies to believers in any other faith, or no faith.
Most of us don't need to advertise our beliefs on our lapels. But if others are sufficiently insecure in their faith that they have to remind everyone, including themselves, what harm? None really if they stick to just wearing a badge. But I bet they don't. The harm is if, as so often, it becomes another nauseating means of proselytising beliefs. That's something the rest of us have no need to do; indeed don't believe in doing. We're secure enough in our beliefs; beliefs which are personal and not to be imposed on others. We don't need lots of other like-minded fools around us to convince us we're right.
Yes, OK, fine if you want to wear a discrete cross, pentacle, Star of David, swastika or whatever on a chain round your neck. But is it just me who finds badges, bumper stickers, prayer beads hanging from driving mirrors etc. somewhat nauseating? And I don't draw the line at religious symbolism. Badges for the Rotary Club, football club, train spotters guild are just as annoying. Why do we need to advertise our allegiances in this way? If we can't spread our faith (whatever that is) by shining example then pretty poor show. Good works and humility, not faith alone.
Who are we to deny such poor benighted souls their comforts? Although can you imagine the outcry if they were forced to wear some identification, as were the Jews in Nazi Germany? They'd be up in arms quicker than a ferret down a drain-pipe.
Maybe I should have a supply of "There is no god" badges made? Or should we all have 42 forceably tattooed on our foreheads?
** There will always be employers who, rightly, ban jewellery for safety reasons. But that is really a side issue.