Know ye that the LORD he is God: it is he that hath made us, and not we ourselves; we are his people, and the sheep of his pasture. (Psalms 100:3)
He is great first and foremost because He is God. There is no other. God’s first commandment to Israel when He delivered them out of Egypt was: “Thou shalt have no other gods before me” (Exodus 20:3), and this was after He had proven Himself more than a match for all the gods of Egypt. He did not make the claim: “I’m the only God because I said so,” although He could have done that; He made the claim based on irrefutable proof. My God is a God of action. He backs up everything He says about Himself.
God is great because He is my Creator: “It is He that hath made us, and not we ourselves.” That in and of itself should inspire awe toward God. Anything I do to assert myself over God, or to claim self-ownership is an act of blasphemy against the great God Who made me.
God is great because He has chosen to be my Lord. “We are His people, and the sheep of His pasture.” I did not seek God out. He chose me! He could have rejected me, passed me over, and He would have been perfectly just in doing so, because I am a sinner both by birth and by choice. But instead, I am one of His through the grace of His Son Jesus Christ. I am a sheep of His pasture, meaning I get to receive the utmost care from a kind and compassionate God. When you consider how this world system is stacked against anyone ever getting to know God, you realize that every salvation is a miracle! My God is great!