What Is God?

Westminster Longer Catechism 

Q. 7. What is God?

God is a Spirit, in and of himself infinite in being, glory, blessedness, and perfection; all-sufficient, eternal, unchangeable, incomprehensible, everywhere present, almighty, knowing all things, most wise, most holy, most just, most merciful and gracious, long-suffering, and abundant in goodness and truth.

Speaking for myself, I prefer a slightly modified answer.  

God is a triune Spirit, who has no substance, no essence, & no esse* & is who infinite with respect to being, wisdom, power, authority, glory, blessedness, and perfection; all-sufficient, eternal, unchangeable, incomprehensible, & who is everywhere present, almighty, knowing all things, most wise, most holy, most just, most merciful and gracious, long-suffering, and abundant in goodness and truth. 

In addition, God is also simultaneously infinitely immanent, immense, & intense.