The Ontological Argument is perhaps the strongest argument for the existence of God. This article will outline the argument and why it is so strong.
Imagine you are walking along a beach, and you stumble upon a watch. You pick up the watch and think :
“I wonder where this watch came from? Who owned it? What is its story?”
You know that the watch could not have been there for ever, as watches are not permanent things, they are optional, watches do not have to exist. When something is optional, in philosophy we call it contingent, meaning that its existence is reliant on something else. Therefore just by the very existence of a temporary, or contingent thing, we know that there must be something that allowed the optional thing to come into existence, this thing is necessary if the contingent exists. Therefore we call it a necessary thing.
If we consider the whole of existence, the universe and everything in it, all of these things are optional, none of them have to exist or are permanent. So just like the watch, by its very existence, we know there must be a necessary thing which allows all these contingent things to exist. This necessary thing is God, it is the permanent source of existence.
If you are interested in the evidence of the existence of God, then I suggest you consider the book I have written on the topic which goes into much more detail and covers many other arguments both for and against.