In classical trinitarianism, such as is confessed in the Nicene Creed, the doctrine of the Trinity states that there are three divine persons, all three of whom are co-essential, co-eternal, distinct from one another, and inseparable from one another. These three persons, as the Nicene Creed sums up, are the one God, the Father, His only-begotten Son, Jesus Christ the Lord, and His Holy Spirit.
Although these three persons are distinct, they are continually in perfect unity of fellowship and communion with one another, and so are inseparable. That the three persons are co-eternal means that they are all eternal, and that they are co-essential means that they all have the same divine nature. The nature that each of the persons has is exactly identical, and so is one and the same. This means that everything that the Father is in His divinity, such as good, perfect, holy, almighty etc. the Son and Spirit also are by nature, because their nature is exactly the same.
The ancient Greek word used by the Nicene Council to describe this is “homoousias”. This gets translated into English in various ways, including “co-essential, “consubstantial”, and “of one nature”. What it literally means is ‘same essence’ (homo=same, ousia=essence). This confession that the three persons all have the same nature has often been summed up as “one essence in three persons”. Again, as the church fathers who framed the Nicene Creed intended this to be understood, this means that the three persons of Father, Son, and Spirit all share the essence, or divine nature.
This confession is good, helpful, and necessary in light of the Arian heresy that attacked the doctrine of the divinity of Christ and the Holy Spirit. Arius argued that the Son was merely a creature, of a different nature or essence than the Father, as we are. Because of this heresy, the church made a special effort to confess their belief in the full divinity of the Son and Holy Spirit in unambiguous terms, especially by introducing the term “homoousias” to unambiguously declare that the Son and Spirit possess the same divine nature as the Father.
To understand this it is important to understand the classical distinction between essence and person. ‘Essence’ refers to nature, the ‘what’ of something. For example in the case of man, when we ask what a man is, we respond that he is human. He has a human nature, which he shares with all other humans. On the other hand, when we talk about ‘person’ we are speaking of the ‘who’ of something. For example, an individual man is a person, such as Peter. So when we speak of one essence in three persons in respect to the Trinity, we speak of one divine nature which exists in three distinct persons.
Unfortunately, as false teachers often do, semi-modalists have twisted this helpful expression of orthodox trinitarianism into something other than what it was intended to say. Instead of thinking of the Trinity as a group of three distinct persons of one essence, they think of the Trinity as a single person who is three persons. They will speak of “one being in three persons”, but this one being is called “he” and otherwise treated like a person in every way except that most refrain from actually using the word “person” for him.
By personifying the divine nature that the persons share, they have created a fourth person of the Trinity. Doing so, of course, constitutes a denial of what the Bible teaches concerning the Trinity, as well as what the early church believed.
Until Christians learn to distinguish between ‘person’ and ‘essence’ properly again, semi-modalism will likely continue to prosper, thriving on people’s confusion and ignorance. It is imperative that Christians recognize the fact that many false teachers have denied the Trinity in this way, and their example must not be followed. They turn the Trinity into a Quadrinity, and so cannot rightly be considered to truly be trinitarians at all. The Trinity has never been one person who is three persons (and so, in sum, four persons) but has always been three and only three persons of one and the same divine nature. So scripture teaches, and so the early church witnesses to, if only we will heed their instruction.