The Trinity is a complex subject, and at times is very misunderstood. Some try to rationalize and fall into error by declaring a type Tritheism, or even Modalism. However, the persons of the Trinity are three persons of one essence. The Trinitarian Godhead is made up of the Father, Son, and Holy Spirit and all within the Trinity are two processions.
To understand this further it is helpful to define what a procession is. Within God there is a two-fold procession as was mentioned earlier. A procession is a general origin of one thing from another. There are two types of procession by which something can come. The first type of procession is known as Ad Extra. This type of procession speaks of something final springing from another. A good example of this would be a human father producing a son. From a theological perspective this is when something springs from God because God is the cause. The second procession type is Ad Intra, and this procession happens when something remains within its principal.
As previously stated, there are two typed of procession within the Trinity. It is also important to note that though different processions are present does not mean that there are different natures. Regarding this Garrigou-Lagrange state, “In the divine processions, for example, there is no diversity of nature (the nature remains numerically the same) but only a diversity of persons according to the opposition of relation” (Garrigou-Lagrange Introduction).
The Father proceeds from no one, and simply is because he has always been. The Father was never created nor begotten, but he does have the operations to know and to will (Lecture Notes). From these operations the Son and the Holy Spirit proceed respectively. To some this may seem that the Son and the Holy Spirit were somehow created? If that is the case then Church History needs to make amends with those who were condemned for heresy, but this is not the case. All are God, and it is from the Father that the Son and the Holy Spirit proceed in God (Garrigou-Lagrange Ch. VII).
The Son proceeds from the Father for all eternity through generation. This type of procession we can see in several places in sacred scripture and also tradition. One such passage is John 1:18 which states, “No one has ever seen God. It is God the only Son, who is close to the Father’s heart, who has made him known” (NRSV). Here we see the generation procession that indicates the special relationship and all-knowing nature of the Son. It is through him that we can have a knowledge of the Father. It is the Father’s principle that is being imitated, and this is because they are one essence (Garrigou-LaGrange Ch. VIII). In the tradition of the church, that is through the councils and writings of the early fathers, there is a constant teaching about the nature of Christ in relation to the Father. That teaching is that the Son is consubstantial and of the same substance of the Father, even though he is begotten. One cannot be begotten and consubstantial at the same time. The Son is the Son because he proceeds from generation.
The procession of the Holy Spirit varies from that of the Son. That is because the third person of the Trinity proceeds from the Father and the Son through spiration and as one principle (Lecture Notes). This is also seen in sacred scripture and tradition. One such verse is John 15:26 which states, “When the Advocate comes, whom I will send to you from the Father, the Spirit of truth who comes from the Father, he will testify on my behalf” (NRSV). This passage shows that the Spirit proceeds from the first two, and this is supported in early church writings from St. Augustine, St. Ambrose, and the Councils of Florence and Lyons.
Garrigou-Lagrange, Reginald. The Trinity and God the Creator. https://www.ewtn.com/library/THEOLOGY/TRINITY.HTM#00, accessed November 27, 2018.