The man deposited the parcel around 0700 GMT at the altar of the Cathedral in central Toulouse during mass before running away, Reuters reported.
The incident also took place a few weeks before the Easter celebrations.
Bomb disposal units intervened on the artisanal device, an acid container, that did not explode due to malfunctions.
Interior Minister Gerald Darmanin tweeted on Friday morning: "Thanks to the rapid intervention of the bomb disposal services, the police operation in Toulouse is over. No casualties. Thanks to all law-enforcement forces."
KI/PR