I personally suspect that the reason the terms have not been defined is because there cannot be a common definition for a phenomenon which is entirely personal. The idea of insisting on an "objective" definition for something that is only found in the subjective universe of one individual at a time is ludicrous on the face of it.

I agree with this. But I don't think it's what causes the problem. Religion is the default position in much of world (certainly in the US, the UK and most of Europe, Africa and Latin America). Religious buildings advertise their faith large and loud (literally loud at times, what with calls to prayer and those lovely jangling bells). Archbishops are embedded in politics. No-one raises an eyebrow at the religionification (I just made that up) of Christmas. I've been to many a wedding and funeral and no-one gives the slightest consideration to the fact that the overtly religious content of the services might sit badly with any atheists present (failure to join in with the lusty hymn-singing can't half make you look a bit of a twat). It isn't thought odd that children scarcely out of their nappies are told to believe that myth is truth (and no more disingenuous comparisons with Santa or tooth fairies, please). A lot of it is more amusing/irritating than important, but the principle is that religion holds sway. It doesn't matter what shape your personal deity takes (Jaysus, given a broad enough definition, I might even have one meself), but you can bank on your religion of choice (or birth, more likely) to megaphone it from the rooftops for you. That's the issue, I think.