This is the only structure still showing those origins, as I understand it. It doesn't mean that it keeps the original style, but that it revels it.

Another view. I don't think "razón" means style, and I don't think it's mere padding. I think Yaiza's "origin" is closer.

I take "razón" here to mean the architectural idea underlying the construction, and I think the word that gets closest to capturing this idea is "conception". In other words, this building was conceived and planned with an architectural system in mind, and that system was mudéjar.

I am deducing this from how I find "razón" used in this sort of context, in expressions such as "razón constructiva" (in relation to Gaudí, for example). In other words, I read it as meaning something like the rationale, the conceptual system, underlying the architecture.

Modern architectural theorists refer to an "architectural system" to refer to something like this, and I think that is also a possibility here:

"The search for a modern architecture was guided by the universal conviction that architecture was grounded in the art of building. Reformers soon had a powerful conceptual tool to assist them through the introduction of a new concept, the 'architectural system'. The phrase 'architectural system' generally designated the integral co-ordination of structure, form and decoration [...]
It was Quatremère de Quincy, an influential figure in the art world, who in the 1820s and 1830s brought the phrase into the mainstream of architectural thought [...] Although Quatremère de Quincy conceded that there were numerous architectural systems, none, he believed, was as satisfactorily developed as that of Greek architecture."
Frank Lloyd Wright and Le Corbusier: the romantic legacy
http://books.google.es/books?id=eEG8AAAAIAAJ&pg=PA2&lpg=PA2&...

Spanish term (edited): que atestigua con certeza su razón arquitectónica mudejar

Since the context makes clear that the reference is to architecture/architectural style, this need not be explicitly mentioned in the English.

In my view, "architectural style" would be awkward here.

Suerte.

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Note added at 1 hr (2011-11-28 09:41:24 GMT)
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It seems significant that, in the posted extract, "mudejar" is in quotation marks. This seems to suggest that the architecture in question "isn't exactly mudejar." If such is the case, it would not make sense to refer to a "conception" or "system."

Mudejar (architecture) style.

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Note added at 3 hrs (2011-11-28 12:13:55 GMT)
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I haven't translated "razón" as I think that can be included in the rest of the phrase. "Razón" is reason, purpose, idea behind, etc.

truly reflects its original (architectural) purpose in a Mudéjar style

I may of course be totally wrong, but again I feel that the author is trying to say that after the restoration it will still be obvious to all that this was/is the chancel. If we take Mudéjar out of the sentence, it makes more sense to me, and I'm wondering if it is in italics because it's almost an aside, additional info, rather than the main focus.

Anyway, this is obviously just an idea to throw out at you, because hopefully the rest of the text will make what he wants to say (should be saying?) clearer

by the way, according to research I have done in the past, Mudéjar should be written with a capital in English and it retains the accent.

Another approach: The only part... that that could be truly considered/ properly or strictly classified as (being) Mudéjar... Mudéjar with or without the accent as determined by your style guidelines for this translation.

Suggestions are adapted from the translation of an interpretation of razón as meaning "que acredite una clasificación como mudéjar" (which seems to fit the context).

en.wikipedia.org/wiki/Spanish_architecture
Mudéjar Style‎: Mudéjar did not involve the creation of new structures (unlike Gothic ... although classified as Mudéjar, are more closely related to the ...; www.scribd.com/.../Elementos-Arquitectonicos-y-Decorativos-...
5 Sep 2011 – ... islamic tradition and the other one can be classified as Renaissance.; contemporaneity.pitt.edu/ojs/index.php/.../5/7 -
Formato de archivo: PDF/Adobe Acrobat - Vista rápida
de IN Sheren - 2011
The question remains, though, as to what extent Mudéjar can be considered Islamic architecture.; Doris Duke Foundation for Islamic Art - Non-Profit ...
www.facebook.com/.../202214289815530 -
Yes, this piece would be considered mudejar. For those not in the know, mudejar is an art historical term that refers to a unique Iberian form of artistic expression, ; www.oldeenglishtiles.com.au/.../andalus-collectio...
They are strongly indebted to the Mudejar from a technical point of view, though they can also be considered Renaissaance from an ornamental standpoint, ...

First the Spanish phrase:
...atestigua con certeza su razón arquitectónica “mudéjar”, Translates in Spanish as ...atestigua con certeza su razón arquitectónica de ser considerada “mudéjar” en arqutectónia,... and therefore my translation in English that calls for a different approach.

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Note added at 22 hrs (2011-11-29 07:01:59 GMT)
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arqutectónia_ typo and repeat word not needed!