There are generally two types of arguments against theism whether we are talking about the Judeo-Christian-Islamic God, the God of Deism, the Hindu pantheon....whatever. They are what I call "rational" arguments and "emotive" arguments.
The "rational" arguments include all scientific, logical and historical problems with a supreme and all-loving Being and include such fields as evolutionary biology, deconstruction of "holy" texts, archaeology, formal logic, etc.
Science in particular has been eliminating the "explanatory" aspect of theism for hundreds of years now -- from Copernicus to Newton to Darwin to Einstein -- all, knowingly or unknowingly, willingly or unwillingly, have had a share in shattering faith. Thus we have a "God of the gaps" with the gaps slowly but uncomprimisingly being filled with naturalistic and non-personal elements.
All the same, I find the "prime mover" argument attractive along with the ever elusive question, "why is there something rather than nothing?" It seems that when we go further and further back in time whether or not we need a "God" to start it all becomes a coin toss of the mind -- maybe yes, maybe no. In short, from a purely rational perspective I believe that the "God hypothesis" remains viable, although weak.
A much stronger arguement against any form of personal god(s) -- in my opinion -- are the "emotive" arguments against a Supreme Deity. Whether the rational arguement for divinity "works" or not, surely the theist runs into very serious problems when we enter the realm of morality and just plain niceness!
What kind of a God would allow a child to get blown to sanguinary pieces after running over a land-mine in a field where she was playing?
What kind of a God would permit an earthquake to obliterate a community and ruin countless lives and families?
What kind of a God would create parasites and bacteria -- some of which are designed to solely live in the human intestinal tract -- and cause a miserable, painful and shameful death?
Who of you would allow retardation in a developing fetus if you had the power to alter the situation?
Suffering. Misery. Bloodshed. Pain. Poverty. Repugnant living conditions. What kind of God would create such a world? Only a devil.
For sure, there are many wonderful, breathtaking and delightful things about our human existence. Some thank "God" for these things. Fine. Well, be sure and blame Him for all the sickness and goulish morbidity that surrounds us as well.
Like it or not, there simply is no way to solve what theologians call the "problem of theodicy" -- why a loving and all-powerful God would allow -- let alone cause! -- evil in the world. All explanations are utter failures to the point where one would conclude that religious folks are simply playing a part in a sick joke.
I think the strongest case against God's existence is the moral one. Suffering and delight. Evil and goodness. Prosperity and poverty. Life and death. These are the hallmarks of a unbelievably complex, yet thoroughly godless world. This is the world we live in: a world of chance, contingency and brilliance. God is dead, but we are alive. Let us be thankful for that.
Bradley