We celebrate a mystery. Trinity means ‘three’, yet God is ‘one’: He is three Persons, one God.
It is a mystery because our language and concepts simply do not stretch that far into the infinite reality God is. We can only try our best to understand what has been revealed by God Himself. This is not a product of the human imagination, nor did we stumble upon this through clever logic; but something that Christ revealed to us in obedience to the Father:
“Father, just as you are in me and I am in you. May they also be in us so that the world may believe that you have sent me”. (John 17:21).
It is a truth which comes from the very mouth of God and in sending us out on our mission to bring Christ to others in a unity of Faith, He does so in a Trinitarian manner:
“Go, therefore, make disciples of all the nations; baptise them in the name of the Father and of the Son and of the Holy Spirit, and teach them to observe all the commands I gave you. And know that I am with you always; yes, to the end of time”. (Matthew 28:19-20).