I have explained the nature and properties of God. I have shown that he necessarily exists, that he is one: that he is, and acts solely by the necessity of his own nature; that he is the free cause of all things, and how he is so; that all things are in God, and so depend on him, that without him they could neither exist nor be conceived; lastly, that all things are predetermined by God, not through his free will or absolute fiat, but from the very nature of God or infinite power. (Spinoza, Ethics)
Most people have not read anything by Spinoza, or even heard of him, which is a shame. If people have heard at all of arguments for the existence of God, they are probably familiar with the five arguments of Thomas Aquinas. Spinoza lays out a very detailed argument for the necessity of the existence of God, but while he is at it he redefines the concept of God in western civilization.
God, according to Spinoza, is “substance,” which meant something different in the 17th century than it does today. By the fourth sentence of the “Ethics”, Spinoza gets right to the definition: “By substance, I mean that which is in itself, and is conceived through itself: in other words, that of which a conception can be formed independently of any other conception.” Or, as God is quoted as saying in Exodus: “I am that I am.“
According to this view, God is existence itself.
Therefore, God necessarily exists.
This may not be the “standard model” (millennia old) notion of God, but many people find it difficult to accept the notion of God as depicted below. However, radical skepticism leads nowhere but to nihilism and an unhappy life.
You have to start with some basic premises or you can never get anywhere.
Take away: What happens if you stop doubting and accept the premise that God necessarily exists?