Theophylact of Ohrid

The words of our Lord seem now to be fulfilled, wherein He said, When I shall be lifted up I will draw all men unto me. For when lifted upon the cross He drew to Him the thief and the centurion, besides some of the Jews also, of whom it follows, And all the people that came together smote their breasts.

Catena Aurea by Aquinas ↗

Bede

And all the multitude of those who came together to this spectacle, and saw the things that happened, returned beating their breasts. That they beat their breasts, because it is a sign of repentance and mourning, can be understood in two ways. For either they mourned unjustly for the one whose life they loved, being killed, or they trembled as they remembered having obtained his death, seeing him more glorified in death. …

On the Gospel of Luke ↗

Bonaventure

Fourth, as to the trembling compunction of the people, he adds: And all the multitude of those who had come together to that spectacle and saw the things that were done, striking their breasts, returned. The striking of the breast is a sign of penitence and compunction, as above in the eighteenth chapter concerning the publican, who "struck his breast, saying: God, be merciful to me a sinner." By this multitude is understood the Jewish people, who, although they saw the truth, as the centurion did, only struck their breasts but did …

Commentary on Luke, Chapter 23 ↗