The 'enchant [quality]' ability defines what an Aura can legally enchant. This is governed by CR 702.5a, which states that the enchant keyword restricts what the Aura can be attached to — the object must satisfy the stated quality at all times the Aura is being attached or is attached.
CR 303.4c further specifies that an Aura's enchant ability acts as a restriction on what it can target when being cast (it must target something that matches the enchant quality), and on what it can legally remain attached to once in play.
Because 'an opponent controls' is part of the enchant quality, your own creatures do not satisfy that restriction. If you attempted to cast such an Aura targeting your own creature, you would be choosing an illegal target, and the spell could not be cast that way (CR 601.2c requires all targets to be legal).
Additionally, if the Aura somehow ended up attached to your own creature (e.g., due to a control-change effect switching who controls the creature), it would immediately be put into the graveyard as a state-based action under CR 704.5n, because it would no longer be legally attached.
Example: You control a Grizzly Bears. You cannot cast Pacifism if it reads 'enchant creature an opponent controls' targeting your own Grizzly Bears — the game won't allow you to choose it as a target, because it fails the enchant restriction.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.