Things escalate when the child becomes increasingly distraught - prompting the priest to tell the crying toddler to "Calm down, calm down" repeatedly before slapping the child on the face.
The 43-second clip, which was recorded during a baptism in France and originally shared on Reddit, shows a priest clutching onto the face of a boy as he sits in a woman's arms and wails.
The priest, who is speaking French, is seen blessing the baby and the child begins crying.
The Daily Mail quickly got wind of the video, but it's not known when the footage was filmed, just that it was captured in a church where the ceremony was conducted in French.
A man and woman, who appear to be the child's parents, looked shocked by the slap.More news: Official Google Podcasts app now available in the Play Store
"I'm going to put water on the forehead, and after I'll embrace the young fellow".
Amid gasps from the congregation, the priest is then seen pulling the child closer and putting his hand over his mouth in a possible effort to stifle his cries.
The bawling baby is being held by its mother when the elderly priest takes a grip of its head with his hand.
The father then pulls the baby away from the priest completely as the video ends.
Twitter users have been taking to the social media platform in droves to weigh in on the horrifying moment, with many tweeting that they would have knocked the priest out. "Calm down", the priest continues before suddenly slapping the infant's cheek.