During the Christological and Pneumatological controversies of the 4th century, one of the most compelling arguments for the Orthodox understanding of the Trinity came from the rite of baptism. It should come as no surprise, then, that St. Ambrose cites the Great Commission in the thirteenth chapter of the first book of his treatise On the Holy Spirit:
Who, then, would dare to deny the oneness of Name, when he sees the oneness of the working. But why should I maintain the unity of the Name by arguments, when there is the plain testimony of the Divine Voice that the Name of the Father, the Son, and the Holy Spirit is one? For it is written: ‘Go, baptize all nations in the Name of the Father, and of the Son, and of the Holy Spirit’ (Matt. 28:19). He said, ‘in the Name,’ not ‘in the Names.’ So, then, the Name of the Father is not one, that of the Son another, and that of the Holy Spirit another, for God is one; the Names are not more than one, for there are not two Gods, or three Gods.
The unity signified by the Name of the Trinity also is the answer to the promise Christ give the Disciples in Matthew 28:20 (the final verse of the Gospel pericope read at Baptism):
Lo, I am whith you until the end of the age.
Since Orthodox Christians are sealed with the gift of the Holy Spirit Himself, Christ is present always through His unity with the Holy Spirit in every Orthodox Christian. Amen.