Continuing our study of the biblical doctrine of the Trinity, which was confessed by the early and medieval church and adopted by the Protestant Reformers, we must further consider how the oneness and the threeness of God are related. It is easy to focus on the unity of the Godhead at the expense of the distinctions between the persons and so miss the individual persons and Their relations to one another. The result can be a practical modalism that sees God as being just like a man who merely holds three different roles depending on the context he is in and to whom he is relating. However true it may be that a man may be a father to his children, a son to his parents, and a husband to his wife all at the same time, that is not what we mean when we say God is simultaneously Father, Son, and Holy Spirit.
Furthermore, we can also find it easy to emphasize the personal distinctions in a way that destroys Their unity. The result here is a practical tritheism that views God as a collection of individual, separable persons united by a common purpose. It is true that a football team is one team even though it is made up of several individual players, but the Trinity is not several individual, separable persons who each contribute Their own talents to the common mission of God.
Though we cannot fully conceive of how God can be one and three at the same time but not in the same sense, the concept of perichōrēsis helps us avoid Trinitarian errors. Perichōrēsis is a Greek term (circumincession in Latin) that refers to the mutual indwelling of the three persons of the Godhead. Perichōrēsis means that the Father is in the Son is in the Holy Spirit. We can distinguish the divine persons, but we cannot pull Them apart. They exist in one another, the Father dwelling completely in the Son and the Holy Spirit, the Son dwelling completely in the Father and the Holy Spirit, and the Holy Spirit dwelling completely in the Father and the Son. We find this concept of perichōrēsis in texts such as John 14:8–11, wherein the Son confesses that He is in the Father and the Father is in Him.
If there is only one divine essence and each of the three persons possesses it fully, perichōrēsis must be true. If each of the persons is all that God is in His essence and there is only one divine essence, the persons must be coextensive with one another. Without the mutual indwelling, They could be pulled apart, and if you pull Them apart, you do not have an undivided, singular divine essence.
Our doctrine of perichōrēsis shows the perfect unity of God. Because the three persons cannot be pulled apart, we know that They are never at odds with one another. The Father is never opposed to the Son or the Holy Spirit. This perfect unity means we can trust that when one of Them speaks, He is revealing the same purpose as the other two. God is not divided against Himself but speaks coherently and in an absolutely trustworthy manner.