The doctrine of the Trinity is the essential doctrine of the Christian faith which we confess. The Trinity is what all other doctrines are built off of. Unfortunately, Evangelicals have not been so good at centering their theology around the doctrine of the Trinity. In the 20th century the doctrine was "recovered" by the likes of Karl Barth, Karl Rahner, Jurgen Moltmann and T.F. Torrance, none of which lay claim to the title "Evangelical".
It is important when considering the Trinity to put "equal ultimacy" on the, to use Cappodocian language, one ousia (being) and the three hypostases (persons). The West, following Augustine, has placed the one being of God at the center of thought. This has led to many problems with Modalistic tendencies. We must confess,with Calvin (though he never said the Holy Spirit was autotheos, I think we can glean this from his insistance on calling the Son such), that the Father, Son and Holy Spirit are all three autotheos.
We also have to center our theology on the Patristic idea of Perichoresis (although John of Damascus was the first to use the term; T.F. Torrance is one of the best modern theologians to shape his doctrine around Perichoresis). Perichoresis is the doctrine that says that the three persons mutually indwell one another in the one being of God, which is why that Gregory of Nanzianzus can say that (and Calvin later):
"..the infinite conjuction of three infinite ones, each God when considered in Himself; as the Father, so the Son; as the Son, so the Holy Spirit; the three one God when contemplated together; each God because consubstantiall one God because of the monarchia. No sooner do I conceive of the one than I am illumined by the splendor of the three; no sooner do I distinguish them than I am carried back to the one. When I think of any one of the three I think of him as the whole, and my eyes are filled, and the greater part of what I am thinking escapes me. I cannot grasp the greatness of the one so as to attribute a greater greatness to the rest. When I contemplate the three together, I see but one torch, and cannot divine or measure out the undivided light (Gregory Nanzianzen "Orations 40.41)."
This is one thing that I absolutely love about the Eastern Orthodox Church. They place the doctrine of the Trinity at the forefront of their theology (see John Zizioulas' Being As Communion for an example). Of course, I do have questions for Eastern Trinitarianism. I am not an adherent to absolute simplicity as Aquinas was. I, like the East, would aknowledge that there is more to God's essence than his existence. The energeiai, I would have no problem affirming. However, I wonder whether or not the emphasis on the energies (in the language of Karl Rahner the "Economic Trinity") and not also on the essence (in the language of Rahner the "Immanent Trinity") of God causes confusion.
I would agree with the so-called Rahner's rule (though not how he probably meant it), that the "The 'economic' Trinity is the 'immanent' Trinity and the 'immanent' Trinity is the 'economic' Trinity. Though, Rahner collapses the economic Trinity into the immanent Trinity and, thus, has panentheistic tendencies. However, I would see this as meaning that the Economic Trinity is a sufficient revelation for being Epistemically Justified knowledge for the Immanent Trinity, or God as He is in Himself.
This is a topic that is much neglected in much Evangelical Dogmatic Theology, and it needs to be corrected. May we see the Triune God in all of His slendor and glory. Amen.