God is a unified being. Moses wrote.
Hear, O Israel: The LORD our God, the LORD is one (Deuteronomy 6:4).
This is also known as the simplicity of God. The idea of God's simplicity has to do with his essence. Although God is a Trinity, he is also a unity – one substance. God is not composed of more than one part or more than one substance.
While human beings are made up of matter and spirit the same is not true with God. Jesus said.
God is spirit, and his worshipers must worship in spirit and in truth (John 4:24).
God, as opposed to humans, is made up of one substance – spirit.
Jesus is God Almighty who became a human being. However in doing so he did not alter his absolute essence - spirit.
The doctrine of the simplicity of God emphasizes that He exists without the need of anything else. For example, there were no forces that came together to form God. This also gives the believer confidence that God will never change. Consequently we can worship him in spirit and in truth as we were meant to.
The simplicity of God means that God is a unified being – He is one essence. God is not composed of a variety of substances. In this sense he is different from humans who are made up of matter and spirit. Jesus is not an exception to this truth. While he took a human body while here on earth he is still absolute spirit in his essence. The simplicity of God reminds us that God needs nothing else to exist neither did he comes about by a number of forces or substances joining together. This reassuring fact will encourage us to worship him as the unchanging God.