Pope Pius XI on the Holy Trinity

The mystery of the most Holy Trinity, which is inscrutable to human understanding, is rich in the splendor of its revelations.  For while the revelation of the mystery does not relieve the impenetrable obscurity it presents to us, nor lift the veil that renders it inaccessible to every created mind, it is nevertheless a revelation that surpasses all others in what it discloses to us of God's being and existence, his essence and his inmost life.  When it comes to these aspects of God, no other mystery tells us so much, nor lends so much assistance to the created intellect in its probe of God.  And yet it remains the inviolate secret of God, infinitely removed from all that is not God.

"Notwithstanding that it conveys only the bare essential of the mystery, what would we, what could we conceive of God without this revelation of his unity and trinity, unity of nature and trinity of Persons?  We should imagine a God isolated and alone, lost in an infinite solitude — a truly frightful, supremely frightful thought, if we may put it so.  It is precisely the mystery of the Trinity that scatters this specter of infinite solitude and, without the least prejudice to God's unity, introduces fellowship, which consists in an infinite communication of infinite goods, and effusion beyond measure of the divine Being itself that goes from the Father who conceives the Son to the Son conceived by the Father and to the Holy Spirit, who proceeds from both.  Truly, an infinite inundation, this ineffable communication of thought and love."

(From A Commentary on the Prefaces and the Eucharistic Prayers of the Roman Missal by Msgr. Louis Soubigou, p. 148)