You may want to read this post on the Mystery of the Trinity first.

Mystery is important when we consider the Holy Trinity, but it’s a “certain” kind of mystery that we use when we refer to the Holy Trinity.

One kind of mystery can act as an escape from an argument.  If we cannot comprehend something, and it is a mystery – we simply do not engage the scornful.

Another sense of mystery is the element of wonder.  Whenever we look into that kind of mystery, it leaves us with a sense of wonder that somehow allows us to grasp something.

St. Gregory the Theologian suggests that a sense of not knowing allows us to say that we have finally grasped something of a subject – that we have contemplated long enough to know that we do not know all there is to know.  In a mystery of escape, the mystery is revealed to the insiders.  We as Orthodox Christians acknowledge that life and faith are not always easy.  We have to have faith and a worship that acknowledges the complexity of our life yet comes into that difficult situation to offer hope, eternal joy, beauty and salvation.  That kind of mystery is associated with eternal glory, beauty, and salvation is healthy for all, and it is what is offered to us in the Orthodox Church.

We can come to know this mystery by observing the early Church Fathers:

Saint Gregory of Nissa talked about how God is “one” and “three,” so people asked him how he could say this and he gave as an answer, an analogy of people.  Imagine three human beings who are three distinct persons but yet they have one humanity so they have one humanity and they share one human nature.

…and so it is with the Father, Son, and Holy Spirit. They share the same nature:

You can ask, “Who is this one?”  We can answer – “This is the Father.”

You can ask, “Who is this one?”  We can answer – “This is the Son.”

You can ask, “Who is this one?  We can answer – “This is the Holy Spirit.”

“What is this one?”  This one is God.

“What is this one?”  This one is God also.

“What is this one?”  This one is God also.

Saint Gregory’s opponents asked how he could make this analogy and retain the belief in one God and he appealed to the mystery of “Personhood.”  He said that we could talk about three people like a construction worker for example – we say that one lays the bricks, one pours the cement, and one drives the nails but somehow when we talk about “Divine Persons” it is different.  Our language falls short of grasping the fullness of what it actually means to be a “Divine Person.”  The Father, Son, and Holy Spirit have EXACTLY the same will and action.  What one does; all do.  So, according to Saint Gregory, a Divine Person must be understood differently than a human person and the analogy can only take us so far.  It helps us to see something of how they can be both one and three, yet in the mystery of Divine Action there is no distinction between the one who lays the bricks, lays the bricks, or drives the nails…all three work as one, sharing one will and one activity.

This was important for the Fathers.  For them, God was not composed of different parts, because God was not composed of a father, a son, and a holy spirit: he wasn’t composed of a nature, a will, and knowledge.

If He was composed of different parts, then those parts would be “prior” to Him.  If we make something like a table, then the legs or the wood used for that table pre-dates the table and that can’t be true for God, because He is The First.  He is the Uncreated One – no one made Him and He is the Source of all life.

It is this mystery and understanding of human language (that can point toward God but never fully grasps what God is), that’s at the heart of our faith.  To understand more fully, we turn to scripture and what has been revealed to us by God Himself, and even then we fall short – and find a sense of wonder.  For example, Saint Paul in his letters will often begin with a grand theological explanation of things, but in the end his words fail him, he falls short of the totality of God’s beauty and wonder, and he closes with “glory to God.”  Praise be to God, because He has given us a wonderful teaching that leads us to awe and the sense of praise.

That is why we as Orthodox Christians come to the Divine Liturgy as the crowning point of our week, because everything we do leads us to this point of praise and worship of God.  All of us participate by standing in glory and honor of the Father, the Son, and the Holy Spirit facing Him – toward the East, toward the way of His Salvation, acknowledging Him as our King.  That is what the Trinity is supposed to do – it is supposed to lead us to a sense of wonder, and to help us understand (even if just a little bit, as much as we are able to grasp), just who our God is and how great and wonderful is the salvation He has given to each one of us.