If I've had to choose no house (say, my opponent played a Restringuntus and Control the Weak naming the same house), can I still use a creature upgraded with Mantle of the Zealot?

"This creature gains, "You may use this creature as if it belonged to the active house.""

I'd guess not, since there isn't an active house for it to belong to?

I'd say you are correct. Since you would be playing the turn with no active house, you couldn't use the upgraded creature.

i agree with blinkingline, and i'd like to note that if something had the "Omni" keyword, it COULD be used if you had "no house" selected.

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The active player may trigger any ”Omni:“ abilities under their control during any of their turns, even if the card with the ”Omni:“ ability does not belong to the active house

I lean towards agreeing with what everyone else has said but that would be a good one to pursue further and get something official. I could see it either way.

I agree with the guy who agrees with the other guy.

I'm going to play devil's advocate and say no. How can it "belong to the active house" if there is no active house?

There is no active house. You must play as though there is no house, when a player is confronted with no legal choice. No house, so the creature can't be used.

1 hour ago, twinstarbmc said:

I'm going to play devil's advocate and say no. How can it "belong to the active house" if there is no active house?

I could see our zany and fun designers saying that the intent of the mantle is to make it so the creature can always be used during a turn, so during a turn with no active house the creature becomes a member of no active house.

Again, not the answer I am leaning towards but I could see them ruling that.

5 hours ago, twinstarbmc said:

I'm going to play devil's advocate and say no. How can it "belong to the active house" if there is no active house?

As much as I agree that this should be the case, the following quote, in red, from the FAQ at the back of the rulebook makes it seem that "no house" is in some way— incomprehensible to me—regarded as a house, while the text on page 10 of the rulebook indicates the opposite. Sigh. Your active house is the absence of a house, which then is made into a house unto itself by what or what may not be unintended phrasing that feel like a contradiction of the rules in the actual document. I'm interested to see how a ruling on this card would go with current precedent being what it isn't.

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If there is no legal choice of house, the player plays the turn with no active house.

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In this case no house is a legal option to be your active house and you must declare no house as your active house

It is likely that phrasing wasn't a priority in the FAQ considering the distinction to be made between the usage of "no active house" in the rules and "no house" in the FAQ. I have no house; it is my active house, seems like a very naturalistic metaphor to be used in card text.