God's existence is necessary in itself.

But God's existence itself is not accessible to the human mind, and the intrinsic necessity of His existence is not comprehensible to the human mind*, and that is why we start with the existence of things-other-than-Him and use those things as the starting point from which to work our way back to God's existence.

* (some arguments such as the Ontological argument work as if those things are accessible to human minds)