"The threeness and the oneness of God are not in the same respect. Although the orthodox interpretation of the Trinity seems contradictory (God is one and yet three), the contradiction is not real, but only apparent. A contradiction exists if something is A and not A at the same time and in the same respect. Unlike modalism, orthodoxy insists that God is three persons at every moment of time. Maintaining his unity as well, orthodoxy deals with the problem by suggesting that the way in which God is three is in some respect different from the way in which he is one. The fourth-century thinkers spoke of one ousia and three hypostases. The problem is determining what these two terms mean, or more broadly, what the difference is between the nature or locus of God’s oneness and that of his threeness… It appears that Tertullian was right in affirming that the doctrine of the Trinity must be divinely revealed, not humanly constructed. It is so absurd from a human standpoint that no one would have invented it. We do not hold the doctrine of the Trinity because it is self-evident or logically cogent. We hold it because God has revealed that this is what he is like. As someone has said of this doctrine:
Try to explain it, and you’ll lose your mind;
But try to deny it, and you’ll lose your soul."
Millard J. Erickson, Christian Theology, 363-367
Actually this is what God is (i.e., Triune) not “what he is like.” Otherwise, I hope the quote proves helpful.