And so one who severely studies the depths of the mystery, receives secretly in his spirit, indeed, a moderate amount of apprehension of the doctrine of God's nature, yet he is unable to explain clearly in words the ineffable depth of this mystery. As, for instance, how the same thing is capable of being numbered and yet rejects numeration, how it is observed with distinctions yet is apprehended as a monad, how it is separate as to personality yet is not divided as to subject matter. For, in personality, the Spirit is one thing and the Word another, and yet again that from which the Word and Spirit is, another.
But when you have gained the conception of what the distinction is in these, the oneness, again, of the nature admits not division, so that the supremacy of the one First Cause is not split and cut up into differing Godships, neither does the statement harmonize with the Jewish dogma, but the truth passes in the mean between these two conceptions, destroying each heresy, and yet accepting what is useful to it from each. The Jewish dogma is destroyed by the acceptance of the Word, and by the belief in the Spirit; while the polytheistic error of the Greek school is made to vanish by the unity of the Nature abrogating this imagination of plurality.
While yet again, of the Jewish conception, let the unity of the Nature stand; and of the Hellenistic, only the distinction as to persons; the remedy against a profane view being thus applied, as required, on either side. For it is as if the number of the triad were a remedy in the case of those who are in error as to the One, and the assertion of the unity for those whose beliefs are dispersed among a number of divinities.
Source: The Great Catechism (New Advent)