As the Gentile Christian movement began to stream from the Jewish establishment that was its original riverbed, the triangle formed between the Father, Son and Holy Spirit began to look confusing to those who believed in the monotheistic base of the faith. The intensity of the struggle to defend the core monotheistic values is difficult to describe. By the time of Justin Martyr (100-165), there was an emphasis on the three, but not as much on the one. Grant quotes Justin’s apology, “we confess the most true God, the Father of righteousness and chastity and the other virtues, untouched by wickedness… we honor and worship him and the Son who came from him and us these things… and the prophetic Spirit.”
Not, in its entirety, the most stalwart of Trinitarian formulas (Justin does put an “army of angels” before the Spirit), the phrase is nonetheless recognizable even to Christians today as part of what we believe. Within Justin’s basic formulation, one can see what will form the fodder for various heresies and struggles to come. Are all three the same in essence? How do the Son and the Spirit come from the Father? Is there a hierarchy? It also became important to establish Jesus as both coming from and being God and having been fully human. Between the efforts of the Cappadocian fathers, Irenaeus, the councils at Chalcedon and Nicea, and countless others, the relationship of the Father, Son and Holy Spirit to one another and to the world was hammered out- both in positive and negative theology (what is and what is not).
The need to define the roles and relationship within and without the Trinity is also rooted in our need (as human beings) to understand the relationship of the Three and the One to ourselves. Beyond theological and historical need, the Trinity must be clarified for our own spiritual and psychological needs. Paul Tillich describes these needs as part of our ultimate concerns. It is enough of a struggle to accept the gifts of grace and faith and understanding the bonds of love between the members of the Trinity, united in one another, can help us understand the love of God, the Father, Son and Holy Spirit, for us. We use specific symbols for the members of the Trinity, which stir in us remembrance, appreciation and comprehension of their actions in, through and for us. Tillich says:
The questions arising out of man’s finitude are answered by the doctrine of God and the symbols used in it. The questions arising out of man’s estrangement are answered by the doctrine of Christ and the symbols applied to it. The questions arising out of the ambiguities of life are answered by the doctrine of the Spirit and its symbols. Each of these answers expresses that which is a matter of ultimate concern in symbols derived from particular revelatory experiences. Their truth lies in their power to express the ultimacy of the ultimate in all directions. The history of the Trinitarian doctrine is a continuous fight against formulations which endanger this power.
Understanding God as Creator, Father or Life-Giver illuminates what we are not able to do for ourselves: bring ourselves into our fullest being, create from nothing and parent with an all-encompassing love. Seeing Jesus in a lamb, an empty cross or footprints stirs in us the recognition of reconciliation beyond what we could do ourselves or even know to ask for. Thinking about the Spirit as a wind blowing through the world, a feeling rising within or an anointing being poured over helps us grasp, in some small way, the reality of the continued action of the Holy in the world.
Though we can recognize them individually with their symbols (that meet our ultimate concerns), together the members of the Trinity also meet our need to understand relationships and mutual love. God sent Jesus, as part of God’s self, to show the way to welcoming arms of a Parent who never forsakes. The Spirit proceeds from them as a promise and a sign that we are not in the world without God. The relationship of the three to one another reminds us God, as the Father, Son and Holy Spirit, is in the same kind of loving, sending, divided and united relationship with us. The doctrine of the Trinity helps us, as believers, to understand the three notes that make up the chord of God. Our faith is not divided among three distinct deities, but enriched by the mystery of a God who is so self-giving that there is no limit, except that of our minds to conceive, to how God can act in, through and for all of creation.
 Tillich, Paul. Systematic Theology, Vol. 3. Chicago, IL: University of Chicago Press, 1963.