Protection from a quality (such as artifacts) grants four specific shields, remembered by the acronym DEBT: the protected permanent cannot be Damaged, Enchanted/Equipped, Blocked, or Targeted by anything with that quality. The 'E' explicitly covers Equipment (CR 702.16b).
Equipment are artifact permanents (CR 301.5). Because protection from artifacts blocks equipping, an Equipment cannot legally be attached to a creature with protection from artifacts — whether you try to attach it via an activated equip ability or any other effect that would attach it (CR 702.16b, 509.2 is not relevant here; see CR 702.16b).
If a creature somehow gains protection from artifacts while already equipped, the Equipment immediately falls off as a state-based action. Specifically, CR 704.5m states that an Aura, Equipment, or Fortification that is attached to something it can't legally be attached to becomes unattached.
Concrete example: You control a Sword of Forge and Flame (an artifact Equipment) and your opponent controls a creature with protection from artifacts. If you activate the Sword's equip ability targeting that creature, the ability is illegal — the creature with protection from artifacts cannot be targeted by the artifact's equip ability, so you cannot even activate it targeting that creature.
In short, protection from artifacts is a hard barrier: the Equipment simply cannot attach, be targeted toward, or remain on any creature that has protection from artifacts.
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.