On this day the Catholic Church observes a special feast in honour of the most profound mystery of our faith, the doctrine of the Holy Trinity. According to this doctrine, there is only one God, but in God there are three distinct Persons, known as the Father, the Son, and the Holy Ghost. In theological language this means that God is one in nature, but three in Persons.
We make no claim that we can give a clear and adequate explanation of this doctrine. It is a mystery-that is, a truth which we accept on the authority of God, but which we cannot understand. We can, indeed, understand that there is only one God. Our reason tells us that it would be a contradiction in terms to say that there is more than one God. The very idea of God indicates absolute supremacy, the possession of all perfections. If there were two Gods,neither could be absolutely perfect because neither would possess the perfections of the other.
But, how this one divine nature can be possessed by three Persons, each distinct from the other, yet each identical with the divinity is entirely above our understanding. However, this is no argument against the reasonableness of believing this doctrine. We accept the statements of our fellow men on matters that surpass our understanding when these persons are intelligent and truthful--for example, the statements of the atomic scientists. Why then should we not accept the statement of the all-knowing and all-truthful God when He tells us that He is one God in three distinct Persons?
The doctrine of the Holy Trinity is the very foundation of Christian belief. In the early Church there were many misunderstandings about this sublime doctrine, so that most of the heresies of the first centuries were centred about this mystery. But the infallible Church pointed out the way to the truth, and hence today in our Catholic theology we have a profound and consistent teaching regarding this doctrine, enabling us to answer all the objections that are raised against it, even though we make no claim to give a positive understanding of its full significance. Catholics should try to familiarise themselves with at least a general knowledge of the Church's theological doctrine on the Holy Trinity.
Whenever you make the sign of the Cross, remember you are making an act of faith in the most sublime Christian doctrine. Try to be recollected, and accompany the words with an interior act of faith that there is one God in three Divine Persons.