Why do we sing/say the “Introit?” What is that anyways?
“Introit” comes from the Latin word for “he enters in.” Originally this began the service, back when confession was said privately the night before. As the minister and assistants entered, a portion of the Psalms was chanted back and forth, so it functioned as a processional hymn. Bar-lines in music had not been invented yet, so they didn’t have hymns with a pulse and beat the way that we sing them today. The advantage of this, of course, was that where churches sing with chanting, they can simply sing the words of Scripture straight from the Bible. Hymns require a bit of paraphrase and interpretation of God’s thoughts, at the very least. When the introit is chanted, it can often have a repeated refrain, called an “antiphon.” The psalm verses sung in the introit are a part of the “proper” of the service. The elements of the worship service fall into two categories: the “proper” and the “ordinary.” The “ordinary” are things that ordinarily happen every Sunday, such as the creed, the Agnus Dei, the Lord’s prayer, and such. The “proper” refers to those things that change from week to week, of which the most important parts are the scripture readings. So in addition to the Old Testament, Epistle and Gospel readings, and the Psalm that is sung, the “Introit” sort of functions like an honorary “fifth reading” at the beginning of the service. It’s purpose is to set the tone of the service and introduce some of the major themes of the day. Today it is popular to use a processional hymn in place of the introit since it does and accomplishes about the same thing (and is easier to walk down the aisle to). However, the introit can also be read responsively as a call to worship: God calls us with His words, and we respond with praise.
From the Large Catechism: For to be baptized in the name of God is to be baptized not by men, but by God Himself. Therefore, although it is performed by human hands, it is nevertheless truly God's own work. From this fact every one may himself readily infer that it is a far higher work than any work performed by a man or a saint. For what work greater than the work of God can we do?