imo

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Note added at 1 hr (2013-02-07 16:30:08 GMT)
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I believe "impious" here= the non-pious people

so, "in whose person" refers to these people

Hereby is banished the error of the impious in whose person it is said:...


The error of impious people is hereby banished since it is said that they...

You really need to give the full context and at least a full sentence to get a full explanation

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Note added at 1 hr (2013-02-07 16:31:22 GMT)
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it might also be said that these people personify impiety

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Note added at 3 hrs (2013-02-07 18:46:10 GMT)
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so, I just found the rest of the context since you asked the question in another forum already

http://forum.thefreedictionary.com/postsm374790_in-whose-per...

and I agree with Thar since it is very similar to what i said already

"Hereby is banished the error of the impious [person];
it is said that in the impious person:......."

so, yes, these are the impious people

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Note added at 3 hrs (2013-02-07 18:50:24 GMT)
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in whose person=is embodied

or these people embody
the essence of these people is

and the context:

Hereby is banished the error of the impious in whose person it is said: We were born out of nothingness, and hereafter we shall be as though we had never been (Wisd. ii, 2); in whose person again Solomon says: One is the perishing of man and beast, and even is the lot of both: as man dies, so do beasts die: all breathe alike, and man hath no advantage over beasts (Eccles iii, 19): that he does not say this in his own person, but in the person of the ungodly, is clear from what he says at the end, as it were drawing a conclusion: Till the dust return to the earth, from whence it came; and the spirit go back to the God who gave it (Eccles xii, 7).

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Note added at 3 days4 hrs (2013-02-10 19:20:56 GMT) Post-grading
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glad to have helped