We know that God necessarily exists and that the act of creation was a sacrifice. We know that God did not create simply to be a mean child because this would mean he lacked something, but how do we know that he didn’t lack someone to love prior to creation?
It is only in the Trinity that we can explain how God is loving yet unchanging. It goes back to the distinction within the Trinity between the persons of God. If God is love, then God must have someone to love. Since creation took place at a point in time, while creation is indicative of God’s love, it cannot be the point where God began to love. Rather, if God is love, then he must have loved eternally, but this would be impossible if God were singular. In a Trinity, however, such a feat is possible.
The Father, being love, must love someone. In this love he must be sacrificial, hoping to gain nothing, but how can this be done absent of creation? The Father could love the Son and in loving the Son he could share everything he is with the Son, that is, he could make the Son equal to himself. What does the Father gain in making the Son equal? Nothing, but he sacrifices any potential selfishness or vainglory in doing so. The Son, being equal with the Father in all aspects, would willingly love the Father as much as the Father loves the Son. His sacrifice would come in following the will of the Father, though he could form his own will (this would later be demonstrated in the death of Christ).
But what is love between two when it can be shared? If a husband and wife refuse to share their love, then are they not selfish? I do not mean by having an open marriage, but by not having children. Or if they refuse to love anyone else because they are too focused on each other, would this not indicate selfishness? Likewise, with the Father and Son it follows that they would have a third person to love so that they may share in equality with this third person. This person is the Spirit.
The Spirit equally loves the Father and Son as they love him. All three are equals, love each other equally, and share in all attributes, save for being begotten and proceeding. We cannot explain how sacrifice exists within the Trinity, only to say that the Father holds the Son and Spirit as equal and that the Son and Spirit obey the will of the Father.
The critic may ask, “If three, why not four? If four, why not five?” If it is necessary for the Spirit to exist so as to avoid the selfish love of the Father and Son, why not have a fourth member of the Godhead? The reason is that once the love has been shared, it cannot become better. In sharing the love with the Spirit, the Father and Son share their love beyond themselves and with someone else. If there were a fourth member then the act would be the same with the fourth as it was with the third. Thus, it becomes superfluous. Beyond this it is a mystery as to why God is only three, for to understand completely why God is three is to understand his essence and we cannot do that.
In all of this, we must understand that the Son and Spirit were not created so that God could love, but rather have existence from the Father and are loved by Him without having been created. If we say that the Son is created, then the Father did not love prior to the Son (for there was no one to love). If we say the Spirit is created, then only selfish love existed between the Father and Son prior to the creation of the Spirit. Thus, all three have existed for all eternity in perpetual love.
Since we know that God is love and he is perfect in this love (that is, unable to gain or add to his love or perfect it) we know that he must be Trinity. In reflecting back to creation we understand that God created us for his own reasons, but in this creation he loved us. However, the love he has for his creation is vastly different than the love he has for himself. Within the Trinity, the persons love each other as equals in all aspects. Within creation, the Trinity loves us as created beings who depend on him for his existence.
Regardless, it follows that since God is the logical conclusion of creation, and creation is the logical conclusion of God’s love, God must necessarily be Trinitarian, otherwise he began to love at a certain point and therefore changed, meaning he is not God. If God was singular before creation then there was no one to love, meaning at creation he began to love. If God was Trinitarian before creation, then he was already loving in perfect form and creation simply became an extension of his love that was not needed, but that he chose to do anyway.