“God plus the world is still God, the Holy Trinity.”
“God minus the world is still God the Holy Trinity,” says Fred Sanders. I would like to add to this the following: “God plus the world is still God, the Holy Trinity.” In other words, creation does not change God, nor does God change God in order to create. No work of God changes God in any sense whatsoever. He is not enhanced by what comes into being or self-enhanced in order to cause things to come into being. Webster says, “[N]o enhancement of God is achieved by the world’s existence.”  Creation is brought into contingent being without any change in the supreme being, who is God, the non-contingent cause of all contingent being. Creation is actuated, that is, caused, or effected. God the Trinity, Father, Son, and Holy Spirit, caused that which has come into being to come into being without any change of being in him. The change goes forth from God’s being and is reflective of that being, however. If change occurs in God due to creation, or anything else, God would not be immutable or simple in any meaningful sense. Confessing trinitarian creation requires that God remains God in the same sense God is God without creation. God just is, full stop. He is God whether creation or not creation; and the God who just is, is the three subsistences of the Trinity eternally. If one posits change in God due to creation, or anything else, one denies divine immutability and divine simplicity, no matter how loudly he attempts to affirm either or both. God is either immutable or not immutable. God is either simple or compounded. There’s no tertium quid (i.e., a third something) in God. Once divine immutability and simplicity are compromised, divine eternity is compromised, as are all other divine perfections. These basic elements of our confessed theology proper are important to remember while discussing creation.
 Webster, God without Measure, 1:91.