Just in case there was any confusion, activating (as jbeers is saying) is when you flip it face up, at the beginning of your turn.
When the ability activates (for instance, Writ) depends on the card. Using your example, you place Unholy Writ in your territory face up (activating the artifact) at the beginning of your turn. When your opponent puts a hero in battle for a rescue or battle challenge, you may then discard Writ to capture the hero (activating the ability).
