A historically orthodox Christian belief is that God has always been and always will (Psalm 90:2, John 8:58). There is no beginning or ending to God. Our ontology, however, must be limited to our concious observations of God. A modern phenomological approach to God demands an explanation of the observable existence of a creator before the creation existed. How can it be observed that God had being, before there were observers of God to concoiusly experience that existence (Others may say that God is the God without being, an interesting conclusion, but not the only option.)? Thus the neccesity of Trinity. Extrapolating, God being omnipresent ceases to have observable being outside of created order if the Trinitarian view of God is denied.
The Trinity becomes absolutely essential to Christian doctrine because, in the very least from a phenomenal view, God, in order to always exist, must always be experientiable. So when the one God of the Christians exists in three peersonalities, He is given to Himself to experience. God is given to observe Himself because He is multiple. One would argue that God could observe Himself if He was not multiple. However, because God is omnipresent, there is no possibility of oberving Himself outside of Himself or in comparison of Himself beyond the existence of another (a creation of His). So, God can be observed to exist eternally because He is more than one.
Therefore, a short allowance towards phonomenology rationally gives that God must be multiple (and it can likely be brought forth that God must be more than two, but a single blog post can only do so much...).
There are immediate applications of this philosophy in systematic and rationalistic theologies and apologetics. Systematics, if they are willing to allow a larger base of evidence than just the Sricptures, can give base to the necessary existence of the Trinity. Rationalists can pose that God is eternally observable, and, thus, eternally existant. Apologists gain a foothold in a postmodern philosophy that lends itself into God's trinitarian existence. All three of these are quick showings that postmodernity is not the enemy (it is not even an enemy system of thought). Rather, postmodernity will allow for a different set of allowances for Christianity that will causally affect the evangelistic impulse of the church for several centuries to come.