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i have written the following, which, if either, is correct: we wash ourselves in the blood of the lamb whomever(whoever)they may be.
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The principle difference between the two pronouns is, very simply, 'whomever' is more formal than 'whoever'. But it needs to be used as the object of a verb, as follows;

Give it to whomever you choose.

In your example i'm not sure of your intended meaning!

Maybe you mean, 'they wash themselves in the blood of the lamb, whomever they are'
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In this case it would refer back to "they" which is subject of the sentence!
In this case, only "whoever" is correct!
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Comments  
What a sentence! Do you think students would use this kind of sentences?
Students: Are you brave enough to let our tutors analyse your pronunciation?
maj, you're right...
It's better to change the whole meaning ***
They should rather not learn those brutal sentences, whoever they are! Emotion: smile
Wherever they are.
I was referring to the characters of the students...
whoever and wherever they are... Emotion: smile
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