God is a Spirit, infinite, eternal, and unchangeable, in his being, wisdom, power, holiness, justice, goodness, and truth.
I have been taught that the superlative attribute of God, the attribute of attributes, is His holiness. Isaiah 6:3 and Revelation 4:8 attest to this when they describe heavenly creatures repeating the description three times to give emphasis: "Holy, Holy, Holy is the Lord God Almighty." God's character is described best through the use of the word holy which shows that He is perfect and excellent.
God's being or existence is most completely described, IMO, with the word eternal or everlasting. God made himself known to Moses and through him to the Israelites with the name Yahweh, "I am." Exodus 3:14-15 records, "God said to Moses, 'I AM WHO I AM.' This is what you are to say to the Israelites: 'I AM has sent me to you.' ' God also said to Moses, 'Say to the Israelites, 'The LORD, the God of your fathers - the God of Abraham, the God of Isaac and the God of Jacob - has sent me to you.' This is my name forever, the name by which I am to be remembered from generation to generation.' " God is. He was and He will be. This is His divine nature. Revelation 1:8 quotes God again: " 'I am the Alpha and the Omega,' says the LORD God, 'Who is, and who was and who is to come, the Almighty.' " God is the Beginning and the End, but has (in himself) no beginning or end. The substance of God is infinite and without limitation and this extends to His existence from everlasting to everlasting.