Why do we always begin each worship service with “In the name of the Father, and of the Son and of the Holy Spirit”?
This introductory phrase, called the “invocation,” is the best way for Trinitarian Christians to begin worship for two reasons: the the power of God’s name, and the presence of Christ. This three-fold name, Father, Son, and Holy Spirit, is the very name that was put on us in our baptism when we became children of God. So we cross ourselves, symbolically binding this name to us, to remind ourselves that God has washed us and made us his own by the power of his Word, binding us to Christ forever. The Christian life begins at baptism, with the name of God, and thus our worship, which serves as a mini-drama or microcosm of the Christian life, begins the same way. This is what it means to “call upon the name of the Lord,” (Genesis 4:26), and, as the second commandment forbids us from misusing the name of the Lord, so we as Christians have the responsibility to use God’s name properly. This is done when we, as Luther says in the Small Catechism, “call upon it in every trouble, pray, praise, and give thanks.” Jesus said, in Matthew 18, “For where two or three are gathered in my name, there am I among them.” So we formally declare that we are gathering in His name, that of the Triune God, because we believe that when we do gather, Christ himself is truly present among us. We don’t worship Jesus as if he were a nice idea or somehow chained to the throne up in heaven incapable of leaving to visit us. Instead, we believe, teach, and confess that through the Word of God and the means of grace, Christ himself is heard speaking to us (the Service of the Word) and seen giving himself to us (the Service of the Sacrament). Many churches today worship Jesus as if he were somewhere else, as if our spirits ascend up into heaven to commune with God there. But in our churches, we believe that instead that Christ himself comes down to us, through the ministry he sent out into the church, to personally bless us with his gifts of forgiveness, life, and salvation.
From the Large Catechism: on the Second Commandment: This command now leads us forward and directs the mouth and tongue to God. For the first things that spring from the heart and show themselves are words [Matthew 12:34]. ...for His name has been revealed and given to us so that it may be of constant use and profit. ...this commandment also applies to right teaching and to calling on His name in trouble or praising and thanking Him in prosperity, and so on. All of this is summed up and commanded in Psalm 50:15, “Call upon me in the day of trouble; I will deliver you, and you shall glorify Me.” For all this is bringing God’s name into the service of truth and using it in a blessed way. In this way His name is hallowed, as we pray in the Lord’s Prayer.