I would not say that ethical behavior is not possible for the atheist or agnostic. It is. A couple of pretty good examples are Bertrand Russell and Jean-Paul Sartre. However, I will have to say that if we take the human lot as a whole, these two men must be seen as exceptions.
Huston SmithWhen there are miles to go before we sleep, altered traits are more important than altered states.
Huston Smith...like a magnetic compass turning north, I always tried to head in the direction of the better, which is the direction to God. ...the directions that appeared to lead away from Christianity led me deeper into it.
Huston Smith