The opposite of anonymity (being unknown) could be fame or renown.
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The opposite of anonymous (unnamed) could be named, or famous.
Another term with an opposite connotation is eponymous(named for something or someone).
To denominate means to give a name to something, so the only antonym would be "anonymity" which doesn't really work. The opposite of a denomination would be something that can't be called by a name, or doesn't identify itself by it's name, or is so universal as to have nothing to distinguish it from anything else. "Catholic" means universal, so in some ways, this is a good opposite for "denominational," but Roman Catholicism is certainly a denomination too.
The opposite (antonym) of opposite is same.
opposite of opposite to opposite from
If the opposite is meant to be the additive opposite and not the multiplicative opposite, then their sum is zero. The reason is that is what defines an additive opposite!
It depends on what you mean by an opposite. Every real number has an additive opposite but 0 does not have a multiplicative opposite.