I think what he's trying to say is that a lot of people(atheists in particular) equate all the religions of the world with Christianity, that is, they seem to think that if you believe in a god of some kind or call yourself religious that also means you must be Christian and that there is no other possible religion you could be. My experience with people who make such assumptions has been that not only do they seem to assume that every religious person is Christian, they seem to assume that every religious person is a narrow-minded, bible-thumping, constantly proselytizing, extremist Christian and thus believe that religion is the cause of all things bad because they think all religious people are like those extremist Christians who want to convrt and control everybody. They don't seem to realize that such people are really a relatively very small minority, they're just a minority that screams very, very, loudly.