Cactus Game Design Message Boards

Do Unholy Writ and Covenant of Noah get discarded the turn after activating them, or do they remain activated until I actually use their ability? 

Remain  active till you use the ability. Note Most limited use cards are like that.
Exceptions:
Burial Shroud
Assyria's Tribute.

I thought I had read somewhere (it was way back in the day, when Warriors first came out) that activating Ark of the Covenant counted as a use even if no one was discarded - even if you didn't attack.

Way back in the day maybe, but not nowadays. Though if you win a battle and the ec is D/c'd it counts.

That's probably it.  For the longest time I couldn't figure out why Unholy Writ was in all the best tourney decks - but I guess I was thinking you only get one activation out of it, even if it doesn't get a use.