It might be better to remember 'kafir' translates in essence to anti-God. Nope it doesn't.. "Kufr" translate to "Those who hides / covers" / "Those try to cover / hide the truth".. The anti of Kufr will be "Those who reveals" / "Those who try to reveal the truth".. The Kufr in Muhammad/Jesus/Abraham times are those who are 'deeply religious'... Among them are high-priests, priests and their (blind) follower... Jesus was convicted to death by "Council of Ulema" and so does Abraham.. both were convicted to death on "Blasphemy Case". Muhammad perhaps received a fair share of death threat from those religious folks.. The Kufr benefited from the lies spread within society and try to do anything include terror, torture and murder to hide the truth and maintain the status quo.. Although they used the terminology like "To maintain the unity of the ummah" or "To protect the believers from shaytanic influence", "To stay true to the path and traditions as set out by our holy/glorious ancestors" etc.. etc.. Principle #1 Truth doesn't need any protection, it just need to be told.. And remember; the "Kufr" on Jesus and John case is a monotheist as well... If you did an objective comparison on how the religious behave from those times and now.. you'll find not much of a difference.. By the time a person is old enough to consent to marriage they are more than old enough to understand the nature of God. So where you might want to start is to ask them if they believe God can be the truth. On contrary I believe those with the purest understanding of the one true God are those who are just being born into this world.. You have to understand their modus operandi first and foremost. The true measure is how liable are they to hold themselves to quranic standards of self - do they stay away from the major evils? The modus operandi of religious people or those who tries to cover the truth is to maintain the 'existing condition' (status quo). Because they gain benefit ($$$,Power and Influence) from the existing condition.. Plus an atheist is more a kafir/mushrik than anything else. If God is defined as a "Sky God" looking down below upon earth.. I myself is an atheist of such god.. I'm also an Atheist of Marduk, Latta, Uzza or other odd deities.... Salam / Peace