From “The Divine Romance” by Venerable Fulton J. Sheen:
The Father loves the Son, Whom He engenders. The Son loves the Father, Who engendered Him. They contemplate each other, love each other, unite in a love so powerful, so strong, and so perfect, that it forms between them a living bond. They give themselves in a love so infinite that, like the truth, which expresses itself only in the giving of a whole personality, their love can express itself in nothing less than a Person, Who is Love. Love at such a stage does not speak; does not cry; does not express itself by words, nor by canticles; it expresses itself as we do in some ineffable moments, by that which indicates the very exhaustion of our giving — namely, a sigh, or a breath. And that is why the Third Person of the Blessed Trinity is called the Holy Spirit or the Holy Ghost.
That breath of love is not a passing one like our own, but an eternal spirit. How all this is done, I know not; but I do believe on the testimony of the Word Incarnate that that Holy Spirit has been sent by God to rule His Church: “But when He, the Spirit of truth is come, He will teach you all truth." (cf. John 16:13) And I believe that the continuous, unbroken succession of the truth communicated by Christ to His Church has survived to our own day; not because of the human organization of the Church, for that is carried on by frail vessels, but because of a profusion of the Spirit of Love over Christ's Vicar and all who belong to Christ's mystical body.
Three in one, Father, Son, and Holy Ghost; three persons in one God; one in essence, distinction of persons — such is the mystery of the Trinity, such is the inner life of God. Just as I am, I know, and I love and yet am one; just as the three angles of a triangle do not make three triangles, but one; just as the heat, power, and light of the sun do not make three suns, but one; just as water, air, and steam are all manifestations of the one substance; just as the form, color, and perfume of the rose do not make three roses, but one; just as our soul, our intellect, and our will do not make three substances, but one; just as one times one times one, does not equal three, but one — so too in some much more mysterious way, there are three Persons in God and yet only one God.
(painting: “La Santísima Trinidad“ by anonymous)