This truth about the Trinity can be summarized in the phrase “ontological equality but relational differences,” where the word ontological means “being.” Both parts of this expression are necessary to a true doctrine of the Trinity: if we do not have ontological equality, then not all the persons are fully God, but if we do not have relational differences (historically called subordination in function), then there is no inherent difference in the way the three persons relate to one another, and consequently we do not have the three distinct persons eternally relating in distinctive ways as Father, Son, and Holy Spirit.

 Grudem, W. (2020). Systematic Theology: An Introduction to Biblical Doctrine (Second Edition, p. 300). Grand Rapids, MI: Zondervan Academic.