No doubt you are already familiar with the concept of the "burden of proof."  (Unless you have already had some unfortunate personal experience with the criminal justice system, just think about the O.J. Simpson trial or any television drama involving the criminal courts.)  The "burden of proof" is on the prosecutor (the D.A.) to convince the jury that the defendant is guilty.  If the prosecutor doesn't present enough convincing evidence, then the defendant is declared "not guilty."

It is the declared intention of atheists to put the burden of proof for the existence of God on the theists.  They don't want to be put in the position of having to prove the non-existence of God.  They know it can't be done.  As was stated in Positive Atheism magazine:  "one cannot prove a negative existential claim (that is, a claim that a thing does not exist)."  For this reason, the distinction between the weak position and the strong position of atheism becomes very important.  With weak-position atheism, the burden of proof falls on the theist.  With strong-position atheism, however, it is the atheist that carries the burden of proof.  Here is how it breaks down:

• The weak-position atheist says:  "I don't believe in God because no one has provided me with any credible evidence that God exists."  This position puts the theist on the defensive.  The theist must present evidence to persuade the weak-position atheist.  
• The strong-position atheist says:  "Absolutely, positively, there is no god."  In response to this dogmatic position, the theistic can say:  "So prove it."  This means that the strong-position atheist must go on the defensive.