The application of these to the Incarnation and Trinity:
The incarnation – since the doctrine of the incarnation arose historically before the doctrine of the Trinity, I will start with this. The doctrine of the incarnation states that Jesus Christ is both man and God, human and divine. Since to say something is a man means to say that thing has the essence of a man, then Jesus must have the human essence, and since to say that something is God is to say that thing has the divine essence, then Jesus must have the divine essence. Consequently, the doctrine of the incarnation means that Jesus has two essences, two natures and two substances – one human, the other divine. What is true of God is true of Jesus and what is true of human nature is true of Jesus. Jesus has a human nature, a body and soul; Jesus also has the divine nature, the divine spirit. The incarnation further means that Jesus has two minds, one human and one divine, and two wills, one human and one divine (although always in perfect conformity.) However, the doctrine of the incarnation teaches that there is only one person. There is one relation, Jesus the son, teaching the gospel, forgiving sins, redeeming mankind. There is only one person because the two substances work as one; they talk as one; they perform miracles as one and they die as one.
The Trinity – the doctrine of the Trinity is the obverse side of the Incarnation. The Incarnation posits two natures in one person whereas the Trinity posits three persons in one nature. Historically, the Trinity emerged between two extremes: Arianism which said that the three persons were three beings each divine (resulting in a kind of polytheism or tritheism) and Sabellianism which said that the three persons were modes or manifestations of one God (resulting in modalism.) The doctrine of the Trinity seeks to find a midway between the two. According to the Trinity, there is only one being: God. There is only one substance and nature, one mind and one will. Furthermore, God is simple. This means that there are no accidents in God (nothing which can be changed without a change to His substance) and consequently whatever is true of God is God’s essence. Thus, God’s actions are also God’s essence. When I type, it is a mere accident to my nature (typing does not define me.) But for God, being simple, what God does God is. So if God loves, He is also His loving. Loving must be His substance if He is truly simple. For this reason, especially in Catholic circles, God has been described as ‘pure action’ as well as pure being. In God, there is no difference between being and action.
It is from this that the idea of personhood develops. The Father is pre-eminently God. The Father is the source of existence and the first person of the Trinity. He is pure spirit. His first action, however, is to know and consequently know Himself. This act of knowing however cannot be accidental to His substance (because in God, there are no accidents.) In this act of self-knowing, he brought forward the idea and image of Himself – and being an act of God which cannot be an accident, it was God’s substance too. This act, while not changing the substance, introduces a relation (the Father and the Father’s self-knowing.) Consequently, according to the definitions above, there are two persons – the Father and the image and self-knowing of the Father. Since the image is generated, begotten not created, it is called the Son (the name of this relation is technically metaphorical.) Furthermore, being perfect, the Father loved the image of Himself, the Son. This is not a romantic love but an intellectual love, an evaluation of the essential good of Himself. Thus, a new act arose, the act of love, which proceeded from the Father and the Son. It too, not being an accident, was of the same being and substance but different relation. Consequently, it too was a person. The three persons of the Trinity therefore are the internal relations of action within the one being. They are all one substance and being but, being different in relation, are called distinct persons. It is important to distinguish these persons from entities; they are internal to the one entity, God, and they arise from the actions within the one substance and they are pure relations. The Father, Son and Holy Spirit may be equally known as Paternity, Filiation and Spiration.
The Perichoresis / circuminsession – Since the persons of the Trinity are of the same being, distinct only in relation, there is never a moment where the Father is and the Son and Holy Spirit is not. To be one person is to be God (because being that person is the act and substance of God, not an accident), however, while the person is distinct from the others in relation, he is not in being. The consequence of this is the doctrine of the Perichoresis or circumincession (or compenetrability) which says that the Father is wholly in the Son and wholly in the Holy Spirit; the Son is wholly in the Father and wholly in the Holy Spirit; the Holy Spirit is wholly in the Father and wholly in the Son.’ Each person dwells in the other (this both has philosophical grounds and scriptural grounds, as Jesus says ‘I am in the Father and the Father in me.&rsquo
Immanent versus economic Trinity – This explanation of the Trinity is described as ‘Immanent’ (‘immanent’ coming from the Latin ‘immanere’ meaning ‘to remain inside&rsquo
. It describes the Trinity only in terms of internal relations, not external relations. This is how it differs from Modalism or Sabellianism which understands the Father, Son and Holy Spirit as manifestations of God to the outside world (the Father as God-creating, the Son as God-begotten-in-Mary and the Holy Spirit as God-dwelling-in-creation.) This immanent explanation however overlooks how the Trinity is involved in the economy of salvation (how Jesus becomes incarnate and uniquely performs the redemption; how the Holy Spirit uniquely descends on the apostles and teaches and comforts them.) The doctrine of the Trinity would stress both the economy and the immanence of the three persons (that is, that the persons are not just
God’s external manifestations in the history of salvation, which would be Seballianism, but internal relations within God.) Hence, it is not wrong to describe the persons as manifestations of God, so long as it is also said that they really correspond to the inner life of God. It is this last point which is most controversial within Trinitarian theology: the extent to which the internal relations of God act independently within creation -- which I myself cannot explain.