Because we agree that such a thing as true debate exists, it follows that we must on some level necessarily agree on the preconditions of debate. In agreeing to debate, he is assuming the authority of reason, and of course sees a correspondence between our rationality and the external world. He assumes that I am arguing a particular position, and that he is countering with another distinct position. He assumes that the position affirming God's existence and the position denying it are mutually exclusive. But all these assumptions are at bottom inconsistent with his declared atheism.
Atheism requires, at the ultimate level, mindlessness or absence of intelligence.
No one is manning the cosmic helm. If there were an ultimate mind, or ultimate intelligence, that ultimate mind would have to be called God. This is something the atheist is happy to grant, and because he denies the existence of God, he thereby cheerfully denies any ultimate intelligence or mind. Here we may have general agreement all round. But there is an insurmountable difficulty.