The Father, the Son, and the Holy Spirit are of one substance and inseparably equal. Their unity is in their essence, their plurality in the persons. The Lord openly showed the unity of the divine essence and the trinity of persons when he said: Baptize them in the name of the Father, and of the Son, and of the Holy Spirit. He did not say in the names, but in the name, by which he showed the unity of essence. But he then used three names in order to show that there are three persons.
In this Trinity can be found the supreme origin of all things, perfect beauty, very blessed joy. As Saint Augustine said in his book on true religion, the supreme origin is God the Father, from whom all things come, from whom proceed the Son and the Holy Spirit. The very perfect beauty is the Son, the truth of the Father, who is not dissimilar to him in anything, whom we venerate with the Father and in the Father, who is the model for all things, because everything was made through him and everything relates to him. The very blessed joy, the sovereign goodness is the Holy Spirit who is the gift of the Father and of the Son; and we must believe and hold that this gift is exactly like the Father and the Son.
When we look at creation, we end up with the Trinity which is of one single substance. We understand one single God: the Father from whom we are, the Son by whom we are, the Holy Spirit in whom we are - the Origin to whom we run; the model whom we follow; the grace which reconciles us.
-Saint Anthony of Padua