Today is the Solemnity of the Most Holy Trinity, the fundamental mystery of our faith: the mystery of God Himself. The only true God is One but He is also Three: He is one God in three Persons. We find this truth very difficult to grasp.
There is no creature in the entire universe that is more than one person. Yet the opposite is true: there are plenty of creatures that are not persons at all. So we can begin to approach an understanding of the Trinity in a negative manner. A dog, for example, is not a person (that is, despite how so many people treat their dogs nowadays): each dog is “one dog” and “no persons”. The same holds true for cats (in spite of what the cat itself may think): each cat is “one cat” and “no persons”. And so on, for every other irrational creature. So we see how it is possible for a creature to possess a nature – “dog”, “cat”, and so forth – but not possess personhood.
Human beings and angels are the only rational creatures in all of creation, and each human being or each angel is one person. From there we move up the hierarchy of being, and the next stop is God, who is uncreated, the origin of all things. And he has one nature, “God”, and three Persons, “Father, Son, and Holy Spirit”: one God in three Persons. The Father is God, the Son is God, and the Holy Spirit is God. But the Father is not the Son, and the Son is the not the Holy Spirit, and the Holy Spirit is not the Father. One God in three distinct persons: the Most Holy Trinity.
There are relations within the Godhead and the wonderful thing is that through our Baptism, we are able to relate with God also. Through Baptism we were brought into the life of God and became: a son or daughter of the Father; a brother or sister (and friend) of the Son, our Lord Jesus Christ; and the Holy Spirit came to dwell within us as in a temple, our constant companion and guide. Reflecting on these things, it would be a fine idea to renew our baptismal commitment today and thank God for having called us to Himself!