In honor of the Feast of the Holy Trinity, a fellow blogger at I Limoni posted a quote from Msgr. Luigi Giussani that reads: "La Trinità vuol dire che la natura dell'Essere è comunità" ('The idea of the Trinity is to say that the very nature of being is "community"').
How often we forget about this element of our Catholic faith: the Holy Trinity. Rarely do we hear prayers addressed to the Trinity as such; rather, we generally approach the Father, or the Son or the Holy Spirit by themselves. In fact, our faith teaches this as well, that certain qualities or characteristics are associated with certain persons. But how often do we stop to think, "What does it mean to say God is three persons in one God?"
Without launching into an extravagant historical analysis of all this, it suffices to say that understanding God to be three persons in one substance goes all the way back to the early Church writers—and particularly the Cappadocian Fathers, like St. Basil the Great. For almost two-thousand years, the Church has interpreted Christ's revelation of the Father and Spirit in terms of person, or 'one in relation to another.' This is evidenced in writers like Tertullian, Athanasius, and in Basil's work especially; and it is a teaching that has endured throughout the centuries.
On the other hand, despite being able to say that there must be some relation/community in God, the idea of how that ought to be formulated has had a much rockier road. There are a myriad of various (and accurate) descriptions of the Trinitarian life, also tracing back to the early fathers, and culminating (more or less) with the Cappadocian formulation. But that doesn't mean it hasn't undergone serious challenge and 'constructive criticism' since then.
One of the best examples is that of St. Anselm of Canterbury (c. 1033AD), who even called God "three substances in one person," rather than the traditional "three persons in one substance." Although Anselm reversed the formulation, he probably did it with a specific goal in mind: to show that the Trinity is a dynamic being; and that each Person (while not a 'substance' in the classic sense) is nevertheless a subsistent relation. But this varied formulation seems to lack a certain clarity (and doctrinal force) that is maintained in the traditional formulation. In fact, Anselm concedes in the end that God is a Trinity "because of the three I know not what" (propter tres nescio quid).
The underlying point, though, is that God is Trinity; and that the Trinity is a personal community. The Father is the Father only because he stands in relation to the Son. And the Son is the Son only because he stands in relation to the Father. These relations are subsistent relations, in that they account for the very identity of the ones in relation (i.e. the divine persons).
Really, struggling with the idea of the Holy Trinity is something eminently Catholic; and something that the greatest minds and saints have been doing now for two millennia. We should continue to do the same thing, and continue to address God under his majestic and solemn title: Sancta Trinitas, unus Deus.