The Concept of the Most Holy Trinity - The Relationship between the Three Persons in One God
A Brief Catechism for Adults - Lesson 3: God and the Holy Trinity
Sheed on the Trinity (Catholic Caucus)
The Father as the Source of the Whole Trinity - Greek and Latin Traditions About the Filioque
The Real Trinity
We believe in one only God, Father, Son and Holy Spirit
Brief Reflections on the Trinity, the Canon of Scripture, and the Protestant idea of Sola Scriptura
Why Do We Believe in the Trinity?
The Holy Trinity
Do check out the Archbishop Sheen link above too. Excellent.
For me, there is no finer explanation of the Holy Trinity than that expressed by the Athanasian (or more precisely, Athanasian-Constaninopolitan) Creed.
My congregation has been known to recite this antiphonally on Trinity Sunday.
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 Substance. 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.
The Father uncreate, the Son uncreate, and the Holy Ghost uncreate.
The Father incomprehensible, the Son incomprehensible, and the Holy Ghost incomprehensible.
The Father eternal, the Son eternal, and the Holy Ghost eternal.
And yet they are not three Eternals, but one Eternal.
As there are not three Uncreated nor three Incomprehensibles, but one Uncreated and one Incomprehensible.
So likewise the Father is almighty, the Son almighty, and the Holy Ghost almighty.
And yet they are not three Almighties, but one Almighty.
So the Father is God, the Son is God, and the Holy Ghost is God.
And yet they are not three Gods, but one God.
So likewise the Father is Lord, the Son Lord, and the Holy Ghost Lord.
And yet not three Lords, but one Lord.
For like as we are compelled by the Christian verity to acknowledge every Person by Himself to be God and Lord, So are we forbidden by the catholic religion to say, There be three Gods, or three Lords.
The Father is made of none: neither created nor begotten.
The Son is of the Father alone; not made, nor created, but begotten.
The Holy Ghost is of the Father and of the Son: neither made, nor created, nor begotten, but proceeding.
So there is one Father, not three Fathers; one Son, not three Sons; one Holy Ghost, not three Holy Ghosts.
And in this Trinity none is before or after other; none is greater or less than another; But the whole three Persons are coeternal together, and coequal: so that in all things, as is aforesaid, the Unity in Trinity and the Trinity in Unity is to be worshiped.
He, therefore, that will be saved must thus think of the Trinity.
Furthermore, it is necessary to everlasting salvation that he also believe faithfully the incarnation of our Lord Jesus Christ.
For the right faith is, that we believe and confess that our Lord Jesus Christ, the Son of God, is God and Man; God of the Substance of the Father, begotten before the worlds; and Man of the substance of His mother, born in the world; Perfect God and perfect Man, of a reasonable soul and human flesh subsisting.
Equal to the Father as touching His Godhead, and inferior to the Father as touching His manhood; Who, although He be God and Man, yet He is not two, but one Christ: One, not by conversion of the Godhead into flesh, but by taking the manhood into God; One altogether; not by confusion of Substance, but by unity of Person.
For as the reasonable soul and flesh is one man, so God and Man is one Christ; Who suffered for our salvation; descended into hell, rose again the third day from the dead; He ascended into heaven; He sitteth on the right hand of the Father, God Almighty; from whence He shall come to judge the quick and the dead.
At whose coming all men shall rise again with their bodies, and shall give an account of their own works.
And they that have done good shall go into life everlasting; and they that have done evil, into everlasting fire.
This is the catholic faith; which except a man believe faithfully and firmly, he cannot be saved.