Martin, I believe, argued there are no "good grounds" of God's existence but theists say otherwise. Which leads to, what are those grounds? According to Wikipedia's Existence of God article, there's several reasons supporting God's existence. First mentioned is the Aquina's Five Ways (Thomas Aquina) of which I simply summarize. First is the motion in the universe meaning we transitioned from potentiality to actuality, so there must be an initial mover because whatever is put in motion has to be done by another thing. Second, it's impossible for a being to cause itself, therefore there must be a first cause, uncaused by itself (which makes little sense to me). Third, Aquina's argument asserts that all beings are contingent, it's possible for them to not exist. At a time, nothing existed but with everything existing now, there must have been a necessary existence who is known as God. Fourth, all things that are good are called into a relation of a standard maximum of goodness, which then causes all goodness. Fifth and lastly, Aquina's argue that unintelligent objects cannot be ordered unless done by an intelligent being, God.
There are many other arguments and examples for believing in a higher power such as God, much more than disbelieving in Him. I find this to be due to human history and the strong focus on the religion that once was.