There are many bodily gestures that Catholics perform at Mass, and one of them that was widely practiced for centuries was the custom of bowing one’s head at the mention of the name of Jesus. Even though it has not been greatly emphasized during the past few decades, it is still honored by many of the lay faithful as well as some priests.
The origin of this custom is primarily inspired by the following words of St. Paul in his letter to the Philippians: Therefore God has highly exalted him and bestowed on him the name which is above every name, that at the name of Jesus every knee should bow, in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the Father. (Philippians 2:9-11)
According to the General Instruction of the Roman Missal (GIRM):
234. There are two kinds of bow, a bow of the head and a bow of the body:
a) A bow of the head is made when the three divine Persons are named together
and at the name of Jesus, Mary and the saint in whose honor Mass is celebrated.
The custom of bowing the head at the mention of Jesus’ name was formally written into law at the Second Council of Lyons, A.D. 1274, convened by Pope Gregory X:
Those who assemble in church should extol with an act of special reverence that Name which is above every Name, than which no other under Heaven has been given to people, in which believers must be saved, the Name, that is, of Jesus Christ, Who will save His people from their sins. Each should fulfill in himself that which is written for all, that at the Name of Jesus every knee should bow; whenever that glorious Name is recalled, especially during the sacred Mysteries of the Mass, everyone should bow the knees of his heart, which he can do even by a bow of his head.
(Sources: Aleteia.org and Church Militant)
Read more in the “Why Do We Do That?” series from Deacon Mike Fritz.