The "atheist" movement keeps shooting itself in the foot by failing to reach a consensus regarding the meaning of "atheism." Allow me to explain.
Individuals who label themselves an "atheist" can be somewhat simplistically divided into (at least) two groups: (1) those who define "atheism" as the mere lack of belief in God, and (2) those who hold the positive belief that God does not exist. Individuals in group 1 include Antony Flew, George Smith, and Michael Martin. Individuals in group 2 include Theodore Drange, Quentin Smith, J.L. Schellenberg, and perhaps William Rowe and Michael Tooley.
"Group 1 atheists" could consistently say that "atheism" means the absence of god-belief and hold the positive belief that God does not exist. Michael Martin is an example of an atheist who does precisely this. On the other hand, "Group 1 atheists" could also consistently define "atheism" the way that they do and lack a belief in the non-existence of God. In other words, an individual in "group 1" might call herself an atheist, but not embrace any arguments for the non-existence of God, including the arguments from evil, reasonable nonbelief, physical minds, and so forth.
Group 1 atheists generally recognize members of group 2 as fellow atheists. Group 2 atheists, on the other hand, tend to say that being in group 1 doesn't automatically make someone an "atheist" (in the group 2 sense). On group 2's view, someone in group 1 could be an "atheist" (in the group 2 sense) but they also might not be an "atheist." It depends.
These distinctions matter because group 2 atheists may not feel represented by a group 1 atheist at all. From the perspective of a group 2 atheist, there is a big difference between someone who says, "I believe God does not exist on the basis of good/strong/conclusive evidence for God's nonexistence," and "I lack belief in both the existence and nonexistence of God." More to the point: this is the problem with Antony Flew, Kai Nielsen, and other group 1 atheists who have purported to represent "atheism" in public debates over God's existence. They are portrayed as defenders of "atheism" (in the group 2 sense, which is how the general public defines "atheism") even though they are not "atheists" in the group 2 sense and, most important, even though they do not defend any arguments for God's nonexistence. (This is not surprising to those who specialize in the philosophy of religion, since these individuals never claimed to hold the positive belief that God does not exist.)
 For an approach to defining atheism that is both more sophisticated and I think ultimately correct, see Ted Drange's "Atheism, Agnosticism, and Noncognitivism." I have deliberately simplified the issue here in order to make a point.