Oblivious to the concept
Mark Anthony Signorelli gets this right : "One answer would be that modern disbelievers, under the influence of their positivist assumptions, simply have no idea what earlier thinkers meant by the concept of God." This is so much the case that when I discuss the standard, millenia-old idea of God here, readers accuse me of "redefining the word"! And this quote is nice, too: "An atheist is just someone who has failed to notice the perfectly evident necessity of God’s existence."