In the Godhead there are three distinct persons, three substances (hypostases), three sources, but one essence, with one nature. They all share the one essence, as well as the one nature. It follows, therefore, that the three persons in the Godhead are of the same essence, coessential, having the same nature.
The fact that the essence of the persons in the Godhead is one, was revealed by Christ in the following words: “Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost” (Matthew 28:19). This bears witness to the fact that the same divine honour – and therefore, the same divine essence – is fitting for the Father and the Holy Spirit, for naturally that which receives equal honour is in fact equal.
Therefore, it is of the same essence and not of a different essence, because if the latter were true, it would be either of better essence or of worse essence. But the better is never given the same or equal honour as the worse. Christ, in His words, ascribes the same divine honour to the Father and to the Son and to the Holy Spirit. Hence, He bears witness to the equality, and consequently to the co-essentiality, of the three persons in the Godhead.
The apostle John wrote, “For there are three that bear record in Heaven, the Father, the Word, and the Holy Ghost: and these three are one” (I John 5: 7). Three and one – the three indicates the number of the persons, and the one, the single one, refers to the one essence. Three persons, but one essence; “and these three are one” -that is to say, one essence.
In discoursing about His relationship with God the Father. Christ said: “Believest thou not that I am in the Father, and the Father in me?” (John 14:10); and, “All things that the Father hath are mine” (John 16:15); and again, “I and my Father are one” (John 10 :30). The Father ha& in Himself the Son and the Son has the Father. That which fills is also filled, and that which is filled also fills. The two are equal.
Moreover, what belongs to the Father is also owned by the Son. Everything that the Father has, belongs to the Son, and everything the Son has, belongs to the Father. Between the two of them exists equality. What belongs to the Father is equal to what belongs to the Son, and vice versa; hence the Son is co-essential with the Father.
The Holy Spirit also is of one essence with the Father and the Son. Hence, He is called the Spirit of the Father. “For it is not ye that speak, but the Spirit of your Father which speaketh in you” (Matthew 10:20). He is called the Spirit of the Son, of Christ, as well: “Now if any man have not the Spirit of Christ, he is none of his” (Romans 8:9). The co-essentiality of the persons involves also, in the case of the Godhead, one, only one – a unique essence of God, which is ever-existent and possesses an eternal existence, unchanging and unchangeable.