Many seem to think this is a tremendous argument (from an atheistic point of view), 'catching out' Christianity and its views on the nature of God. However, God is not subject to fate, or limited roles within the cosmos. The Orthodox Doctrine of God specifies that God's ontology is subject to His perfect free-will... Why is He good? Because He wills good. Why does He not lie? Because He wills to be honest. Why does He exist as Trinity? Because He wills it. The fact that God does not do something in one instance or another, is not because He does not have the ability, but because that is His good will.
An argument based on strict logical word games can render the idea of 'all powerful' or 'omnipotent' self-defeating. This becomes very clear when one considers the classic question 'Can God create a rock so big that He can't lift it?' For a Christian, calling God all-powerful simply means that all power and authority are God's. The answer to this question would be that God cannot overpower Himself, and this fact is not confounding, but it stresses the point that there is no greater power than God, so much so that one is forced to pit God against Himself in order to find His equal.
With guidance from : http://www.pravmir.com/top-10-common-atheist-arguments-fail/
( Where you can find answers to all these refutable atheist 'arguments' )