In the very highest place, deep within a mansion
dwells a family perfectly united, loving and devoted.
Beyond past, beyond present, the three Persons are one;
penetrating heaven, penetrating earth,
the one family is three!
- Wu Li, SJ
Go here for the rest.
And on Trinity Sunday one would be remiss not to feature the Athanasian Creed:
Whosoever will be saved, before all things it is necessary that he hold the Catholic Faith.
Which Faith except every one do keep whole and undefiled; without doubt he shall perish everlastingly.
And the Catholic Faith is this:
That we worship one God in Trinity, and Trinity in Unity;
Neither confounding the Persons; nor dividing the Essence.
For there is one Person of the Father; another of the Son; and another of the Holy Ghost.
But the Godhead of the Father, of the Son, and of the Holy Ghost, is all one;
the Glory equal, the Majesty coeternal.
Such as the Father is; such is the Son; and such is the Holy Ghost.
...So there is one Father, not three Fathers; one Son, not three Sons; one Holy Ghost, not three Holy Ghosts.
in this Trinity none is before, or after another; none is greater, or less than another.
But the whole three Persons are coeternal, and coequal.
So that in all things, as aforesaid; the Unity in Trinity, and the Trinity in Unity, is to be worshipped. He therefore that will be saved, let him thus think of the Trinity.

