Well, the first reason is because that is what we are supposed to do.
There is a Church document called the General Instruction of the Roman Missal (or “GIRM”) that outlines the basic structure of the Mass. In para. 137 we read:
- “The Creed is sung or recited by the priest together with the people with everyone standing. At the words et incarnatus est (by the power of the Holy Spirit . . . became man) all make a profound bow; but on the Solemnities of the Annunciation and of the Nativity of the Lord, all genuflect.”
So, just to refresh your memory, here is the Nicene Creed as it is said in Mass, up to the part where we are supposed to make a profound bow:
- We believe in one God, the Father, the Almighty, maker of heaven and earth, and of all that is, seen and unseen.
We believe in one Lord, Jesus Christ, the only Son of God, eternally begotten of the Father, God from God, Light from Light, true God from true God, begotten, not made, one in Being with the Father.
Through him all things were made.
For us men and for our salvation, he came down from heaven:
by the power of the Holy Spirit he was born of the Virgin Mary, and became man.
You begin the bow at the words “by the power ...” and rise once you have said “became man.” Note that a “profound bow” is a bow at the waist, in the manner of the Japanese when they greet people. This is more demonstrative then the simple head nod.
The reason the GIRM calls us to bow at that point in the Creed is because it is at that point when we express our belief in the most fundamental mystery of Christianity: That in Jesus Christ, God actually became man, born of a woman by the power of the Holy Spirit. This is the Incarnation, the most sacred moment in all of human history. Therefore, we bow in honor of that event.
In the Mass, our gestures and postures should always reflect what we believe at each moment in the Mass. Since the Incarnation is so central to what we believe as Christians, the profound bow becomes an important way of outwardly expressing that belief.