Sorry,
The word I should have used is "essence" or "being". I'm taking this from the Nicene Creed which uses ousia, and declares the Father and the Son to be homoousian, or "of one being or essence".
From what I'm reading, it would be difficult to apply the word "substance" to God as God does not have accidents, even though the Catholic Encyclopedia defines substance as "signifying being as existing in and by itself".
Either way, by substance I mean to indicate that the Son and the Father are of the same "being".
Though not found in the New Testament, I think this word (homoousian) is consistent and expresses with deeper clarity the message of Scripture.
Being or essence is a very difficult concept to describe. The Encyclopedia defines essence, vaguely, "as properly described as that whereby a thing is what it is".
In other words, this is to say that the Son and Father, in essentially "what they are" or what constitutes their existence, are one. Shed any further light? Probably not.
The Trinity is, for us, a matter of faith and divinely revealed truth. This we see as perfectly harmonious and, infact, even demanded by the Epistle of John which teaches that "God is love".
Accordingly, the Trinity is really the only theological model in which we can affirm this to be a truth of God's being. God is love. Love is not impersonal, it is not a "force", but a reality between persons. In light of the Trinity we can say that God is not simply, from time eternal, loving himself in the kind of self-intoxicated or narcissistic sense. God is love because his being is, in fact, a society of persons. The Father eternally makes a gift of his self to the Son, and the Son to the Father and together with the Spirit. Thus (and some of this may be my own theological speculation) the Godhead is like three poles leaning on one another for support. Each holds nothing back because each person makes a total gift of himself, yet each stands firm by the total gift of the other.
This communion of love, this society of persons is what constitutes God's "being", his central reality. It is the deepest mystery. And from this all love is drawn. In the image of God, all human beings are drawn into communion with one another, more so the more we set our gaze to God, who images love in his very being. Because God is a society of persons, all human beings are drawn into relationships as well.
Now as each of us is called to love, that is, as we are called to make a total gift of ourselves, we realize that we inevitably fail to take the full step. On account of sin we selfishly keep ourselves and refuse to enter fully into the vulnerability of love.
Thus the Son descends to us, becoming human he takes us into him, and makes a complete gift of his self to the Father so that we, sharing in him, can become part of his self-giving. We see that Christ has modeled perfectly the "vulnerability of love" in his passion and Cross. We see his gift of self on Calvary, furthermore, that self is still being given in the Holy Eucharist and in the entire life of the Church. Thus the call of every Christian is to actually enter, through the vulnerability of love in Christ, into the Trinitarian dynamic.
Thus we say that love is the founding principle of the universe. It kind of "pours out of what God is", so to speak. Human beings participate in God then through love, he is what draws us all together as the only being that is complete in himself.
Whew, heavy stuff. Hope that might help!
