As we conclude our short study on the doctrine of the Trinity, it will be important to discuss some of the terminology developed to summarize the biblical teaching on the nature and existence of God. There are two key terms to know in order for us to confess the historic, orthodox Christian doctrine of the Trinity.
First, we have the Greek word ousia, which is translated into English as “essence.” The essence of something is that which makes it what it is, the attributes that define a particular entity. There is a human essence, made up of things such as mind, will, soul, and physical embodiment that make human beings human beings. There is a dog essence that is made up of things such as physical embodiment and a canine mind, but a dog essence is different from a human essence. Multiple human beings exist—Bob’s essence and Jim’s essence and Susan’s essence are all fully human, but Bob and Jim and Susan do not possess the same mind, will, or body even though each person has a human mind, a human will, and a human body.
God has an essence as well—the divine essence. But the divine essence is unique, for God is the only divine being (and thus, God actually is His essence). Further, while we may distinguish His divine attributes, we may not separate them. In the essence of God, His mind is His will is His power and so on. Now, when we say God is the divine essence, we are really saying that the Father, Son, and Holy Spirit are all divine in equal measure. There are real differences between Father, Son, and Spirit, but the differences do not touch what makes God God. Moreover, the essence of the Father is identical to the essence of the Son is identical to the essence of the Spirit. So, for example, each person does not have His own unique mind, will, or power; They all have the same mind, will, and power. Thus, the church has confessed that the Father, Son, and Spirit are homoousios, “of the same essence.”
The second key term is hypostasis, which we normally translate as “person.” It is a bit harder to understand this term because it does not mean the same thing as our modern concept of personhood. A person, in Trinitarian theology, is defined by a personal property that distinguishes Him from another person without dividing the divine essence or producing three gods. So, while the Father, Son, and Holy Spirit share the same essence fully, there is a real, eternal difference between Father, Son, and Spirit that pertains not to the divine attributes but to the way the three exist in relation to one another.