The Invalidity of Atheism Thanks Agent Smith - neither theism nor atheism are arguments but rather beliefs. If requests/demands are made for justification then an argument might follow. Theism might be partly justified by reference to empirical experience. Many people claim to 'sense' the existence of a God. This is not the kind of formal empirical evidence used to justify scientific propositions but nevertheless it is perhaps some kind of psychological evidence of something. On the empirical side, simply because atheists have not shared this religious experience, that would not justify the statement that there was no object to this experience. Just because I don't see it does not mean that it does not exist. There have been plenty of arguments for the existence of God, dating back to Aristotle and carried forward by Aquinas. Are any of them valid? Logical arguments can be less than valid without necessarily being invalid. They can be persuasive, plausible and intriguing. Descartes ontological argument is a case in point.
Most religious people however would argue that their beliefs were a matter of faith, not subject to empirical or rational validation. If they were validated there would be no basis for faith. If you want to understand the nature and importance of faith I would recommend two sources, the first, the gospels and teachings of Jesus, and the second Wittgenstein's book 'On Certainty' (as well as the Philosophical Investigations).