What is Trinity Sunday?
Trinity Sunday is one of our Christian days of celebration. It is different from many other celebrations because it does not focus on an event, such as the birth of Christ (Christmas), the resurrection (Easter), or the coming of the Holy Spirit (Pentecost). Rather, Trinity Sunday focuses on a doctrine. We step back from all the events, look at them together, and draw a conclusion: the ONE God is also THREE Persons. The Westminster Confession states, "In the unity of the Godhead there be three persons, of one substance, power and eternity: God the Father, God the Son, and God the Holy Ghost. (II.2. 3)
Admittedly, this doctrine sounds strange. How can God be both ONE and THREE? The early church wrestled with this question for at least the first two centuries of its history. The answer that was finally settled on is that we don't know how God does it, only that God does. We are inescapably led to the conclusion that God is a Trinity because of God's self-revelation.
The best way to think of the Trinity is by remembering the basic story of God: Jesus came into the world, sent by God. He called God his Father. Jesus died and rose again and ascended into heaven and is worshiped, proving he himself is also divine. He is God the Son. Then Jesus sent the Holy Spirit, who is God among us and within us. So, when we put it all together, we end up with three distinct persons -- Father, Son, and Holy Spirit -- each of whom is divine. Yet we believe in only one God. Hence the doctrine of the Trinity: Three Persons (tri), One God (unity).
Importantly, in the theological treatises of the early church, the fathers labored to point out that the doctrine of the Trinity was not a covert form of polytheism. We do not believe in three Gods. That would be called "tritheism." On the other hand, we do not believe that the Father, the Son, and the Holy Spirit are merely different masks for one God. That would suggest that God was acting out roles and confusing us. Rather, we believe in One divine nature shared among three persons who distinguish themselves from each other. The fathers devised the doctrine of the Trinity to help us to stand in solidarity with the monotheism of the Jewish Scriptures and yet also affirm that Jesus and the Holy Spirit are divine.
Okay, enough mental gymnastics. Why do we celebrate the Trinity, even though it is so mysterious and hard to explain? We celebrate the Trinity because it assures us that all our experiences of Christ are actions of God toward us. The Father so loved the world that he sent his Son. That proves God loves us. The Son died and rose again to forgive sins and open the way to eternal life. That proves God forgives sins. The Holy Spirit dwells within and among us. That proves God dwells within and among us. We believe our experiences of God are just that -- experiences of God.
In short, we celebrate the doctrine of the Trinity because it is our way of affirming that God -- Father, Son, and Holy Spirit -- has engaged us with love, redemption, and intimacy.