I'm not sure why the focus is on Abraham... Why not Adam? Or Eve?
But to an outsider, it seems Judaism, Christianity, and Islam have a lot of the same 'players'. Adherents to all three of those faiths will know who Abraham is, at the very least. Mention him to someone outside of those faiths(that was never a part of those faiths), and they likely won't know who you're talking about.
Same with Adam and Eve, or Jacob, or... heck, I'm pulling at names, because I don't know very much, but I hope you get where I'm coming from.