18 Now as to what that mode, or immoderateness rather, of blasphemy is, what that particular blasphemy, and what that word against the Holy Ghost, the order of my discourse requires me to say what I think, and not to put off any longer your expectation which has been so long but so necessarily deferred. You know, Dearly beloved, that in that invisible and incorruptible Trinity, which our faith and the Church Catholic maintains and preaches, God the Father is not the Father of the Holy Spirit, but of the Son; and that God the Son is not the Son of the Holy Spirit, but of the Father; but that God the Holy Spirit is the Spirit not of the Father only, or of the Son only, but of the Father and the Son.
And that this Trinity, although the Property and particular Subsistence of each person is preserved, is yet, because of the undivided and inseparable Essence or Nature of Eternity, Truth, and Goodness, not three Gods but One God. And by this means, according to our capacity, and as far as it is granted us to see these things “through a glass darkly,” especially being such as we now are, there is conveyed to us the idea of Origination in the Father, Nativity in the Son, and the Communion of the Father and the Son in the Holy Spirit, and in the Three Equality.
By That then which is the Bond of communion between the Father and the Son, it is Their pleasure that we should have communion both among ourselves and with Them, and to gather us together in one by that same Gift, which One They both have, that is, by the Holy Spirit, at once God and the Gift of God. For in This are we reconciled to the Divinity, and take delight in It. For what would the knowledge of whatever good we know profit us, unless we also loved it? But as it is by the truth that we learn, so is it by charity that we love, that so we may attain also to a fuller knowledge, and enjoy in blessedness what we know.
“Love moreover is shed abroad in our hearts by the Holy Ghost which is given unto us.” And because it is through sin that we are alienated from the possession of true good, “Love covers a multitude of sins.” So then the Father is Himself the True Origin to the Son, who is the Truth, and the Son is the Truth, originating from the True Father, and the Holy Spirit is Goodness, shed abroad from the Good Father and the Good Son; but in all Three the Divinity is equal, and the Unity Inseparable.
Source: Sermons on Selected Lessons of the New Testament (New Advent)