2 And if we consider the places, I say with confidence (though in fear I say it), that the Trinity is in a manner separable. When Jesus came to the river, He came from one place to another; and the Dove descended from heaven to earth, from one place to another; and the very Voice of the Father sounded neither from the earth, nor from the water, but from heaven; these three are as it were separated in places, in offices, and in works. But one may say to me, “Show the Trinity to be inseparable rather.
Remember that you who art speaking art a Catholic, and to Catholics are you speaking.” For thus does our faith teach, that is, the true, the right Catholic faith, gathered not by the opinion of private judgment, but by the witness of the Scriptures, not subject to the fluctuations of heretical rashness, but grounded on Apostolic truth: this we know, this we believe. This though we see it not with our eyes, nor as yet with the heart, so long as we are being purified by faith, yet by this faith we most lightly and most strenuously maintain— That the Father, Son, and Holy Spirit are a Trinity inseparable; One God, not three Gods.
But yet so One God, as that the Son is not the Father, and the Father is not the Son, and the Holy Spirit is neither the Father nor the Son, but the Spirit of the Father and of the Son. This ineffable Divinity, abiding ever in itself, making all things new, creating, creating anew, sending, recalling, judging, delivering, this Trinity, I say, we know to be at once ineffable and inseparable.
Source: Sermons on Selected Lessons of the New Testament (New Advent)