This excerpt on the Trinity as Mystery from the dialogue On Divine Providence by Saint Catherine of Siena (Cap 167, Gratiarum actio ad Trinitatem) is used in the Roman Office of Readings for the liturgical memorial of St. Catherine of Siena on April 29.
Eternal God, eternal Trinity, you have made the blood of Christ so precious through his sharing in your divine nature. You are a mystery as deep as the sea; the more I search, the more I find, and the more I find the more I search for you. But I can never be satisfied; what I receive will ever leave me desiring more. When you fill my soul I have an even greater hunger, and I grow more famished for your light. I desire above all to see you, the true light, as you really are.
I have tasted and seen the depth of your mystery and the beauty of your creation with the light of my understanding. I have clothed myself with your likeness and have seen what I shall be. Eternal Father, you have given me a share in your power and the wisdom that Christ claims as his own, and your Holy Spirit has given me the desire to love you. You are my Creator, eternal Trinity, and I am your creature. You have made of me a new creation in the blood of your Son, and I know that you are moved with love at the beauty of your creation, for you have enlightened me.
Eternal Trinity, Godhead, mystery deep as the sea, you could give me no greater gift than the gift of yourself. For you are a fire ever burning and never consumed, which itself consumes all the selfish love that fills my being. Yes, you are a fire that takes away the coldness, illuminates the mind with its light and causes me to know your truth. By this light, reflected as it were in a mirror, I recognise that you are the highest good, one we can neither comprehend nor fathom. And I know that you are beauty and wisdom itself. The food of angels, you gave yourself to man in the fire of your love.
You are the garment which covers our nakedness, and in our hunger you are a satisfying food, for you are sweetness and in you there is no taste of bitterness, O triune God!