Happy Trinity Sunday!

What is the doctrine of the Trinity and why does it matter?  Dr. Robert Stackpole answers.  Excerpts:

"When I am reading the Diary of St. Faustina, I find great comfort and consolation in all that Sr. Faustina writes about the compassionate Heart of Jesus and His merciful love for us. But when she starts talking about the Holy Trinity, my mind goes kind of 'blank,' and I really do not know what she is talking about."

If that has been your experience too, then know that you are not alone. In fact, many millions of Catholics have never really been brought "up to speed" on what the Church teaches about the Holy Trinity, and why it is so important — indeed, why, in many ways, it is the central doctrine of our faith. When St. Faustina writes about the Trinity, she assumes knowledge of Trinitarian doctrine that few of us possess, and this can make those passages in her Diary seem somewhat foreign and opaque to us. But fear not: With the help of the Holy Spirit, over the next few weeks, we shall ponder the mystery of the Trinity together, and thereby draw a little closer to St. Faustina in mind and heart.

...One time, after receiving Holy Communion, she [St. Faustina) began to understand that the entire Holy Trinity came to dwell within her soul:

Once after Holy Communion, I heard these words: You are our dwelling place. At that moment I felt in my soul the presence of the Holy Trinity, the Father, the Son, and the Holy Spirit. I felt that I was the Temple of God. I felt that I was a child of the Father. I cannot explain all this, but the Spirit understands this well (451).

Now, you might be thinking to yourself, "That is all very beautiful, but I still don't get it! What does the Trinity really mean? How can God be One in being and essence, but Three Persons at the same time? And what difference does it really make if we believe all this or not?"

Well, Sr. Faustina wanted to know more, too! Listen to what she wrote near the very start of her Diary:

On one occasion I was reflecting on the Holy Trinity, on the essence of God. I absolutely wanted to know and fathom who God is. ... In an instant my spirit was caught up into what seemed to be the next world. I saw an inaccessible light, and in this light what appeared like three sources of light that I could not understand. And out of this light came words in the form of lightning which encircled heaven and earth. Not understanding anything, I was very sad. Suddenly, from this sea of inaccessible light came our dearly beloved Savior, unutterably beautiful with His shining Wounds. And from this light there came a voice which said, Who God is in His Essence, no one will fathom, neither the mind of angels nor of man. Jesus said to me, Get to know God by contemplating His attributes. A moment later, He traced the sign of the cross with His hand and vanished (30).

So, right from the start, Jesus was telling her that no one can ever completely fathom the mystery of the Trinitarian Being of God, but that if we contemplate God's attributes, we can at least begin to understand it. And as we have seen, that is precisely what Sr. Faustina did: The more she contemplated the compassionate love in the Heart of Jesus, and the more she appreciated the "glowing center of love" in God and the "burning center of God's love," especially manifested in the Holy Eucharist, the more she began to appreciate what it means to say that God is Three Persons in One Being or Essence...

For more, see here.