What this means is that God is one in substance (nature, essence), but there are three co-equal Persons who possess the same substance, and so share in the one thinking, willing and activity of God. So we can say that there is a “community” in God, unlike any community we know, but still a union of three Persons.

The fact that there are three Persons in one God has been revealed to us by Jesus Christ. For he prayed to the Father and addressed him as “My Father,” which means that Jesus is the Son; and he also told us about the Holy Spirit whom he would send on the Church after his ascension into heaven (see John 14, 15 and 16). That there are three Persons in God is brought out by the baptism formula in Matt. 28:19 and by the invocation of St. Paul in 2 Cor. 13:13 which is used at the beginning of each Mass. In fact, the three Persons are linked together in about 40 different passages in the New Testament.

The Trinity has to do with the inner life of God. The only way we can know something about it is if God tells us and he has done that in the New Testament and in Church tradition. The philosophers like Plato and Aristotle never had a clue as to the inner nature of God. They knew about his existence and his perfections which are mirrored in the created world, but those things reflect the substance of God which is common to all three Persons; human reason could never see that God is Triune from a consideration of creatures because creation is common to all three Persons. The principle here is that all acts outside the Trinity are common to all three Persons, including the infusion of sanctifying grace and the indwelling of the Trinity in the soul of the justified person.

How can God be both three and one? Is that not a contradiction? It is not a contradiction if we make the necessary distinction that three and one are not meant in the same sense. The orthodox answer to this objection is to say that God is one in substance and three in Persons. In this short space I cannot adequately explain all these ideas; here I am giving a mere summary. The theologians explain the inner life of the Trinity on the basis of the two activities of all spiritual beings—thinking and willing. Thus in God there are two activities which are called “processions,” thinking and willing. So the Son is eternally generated by the thinking of the Father, and the Holy Spirit is “breathed forth” by the intense love that exists between the Father and the Son; he is said to be the “kiss” of the Father and the Son.

The two processions give rise to the three Persons and the Persons are said to be subsistent relations. This is necessary in order to maintain the unity and simplicity of God. If this editorial sends you to your books, fine; you might begin with the new Catechism (##232-267).

Please note that most of the prayers of the Church are directed to the Father, through the Son, in the unity of the Holy Spirit. When we pray we are praying to Persons and the love of the Trinity for us is so great that the three Persons dwell in the souls of those who are in the state of sanctifying grace (see John 14:23).