- In the literature, there are usually two varieties of Atheism that are distinguished from each other: weak (negative) atheism and strong (positive) atheism. Weak atheism is the lack of belief in the existence of God: weak atheists feel that those who believe in God carry the burden of evidence. Strong atheism is the belief that God does not exist: strong atheists feel that the evidence points to God not existing. (Note that a lack of belief in X does not necessarily entail a belief in Not X.)
- One cannot both believe that God exists and not believe that God exists. They can, however, believe neither.
- Now, an agnostic is, according to Merriam Webster, "one who is not committed to believing in either the existence or the nonexistence of God or a god". Agnostics do not believe that God exists, nor do they believe that God does not exist.
- Therefore, by 2 and the definition of agnosticism, agnostics do not believe that God exists.
- Therefore, by 4 and the definition of weak atheism, agnostics are weak atheists.
So in summary: one can either believe that God exists, or not believe that God exists (although the latter does not entail believing that God does NOT exist). Because agnostics do not hold any particular attitude about God's existence, they do not believe that God exists. Therefore they are weak atheists.
Note on "belief" and "knowledge": "agnosticism" strictly means "not knowing". So strictly speaking, religious people and atheists could be agnostic, even though they have beliefs on the matter, if they do not feel that they know whether god exists. However, the term has come to mean not believing anything one way or the other (as can be seen in the definition). With this meaning, my argument holds.