Should words be capitalized for being religious terms?
Not necessarily. It depends on whether they’re considered proper names.
For example, church, communion, atheist, agnostic, and spirituality are not proper names because they’re not recognised entities.
In contrast, the Church of England is a proper name because it refers to an institution and should therefore be capitalised. However the church is not capitalised when you’re simply referring to a building rather than to the institution itself.
Holy Communion should be capitalised as a proper name; however, communion used as a general term should not be, because it isn’t a proper name.
Style guides may differ on this point—because it is strictly a matter of stylistic preference, not logical superiority—but most of the ones I checked don't address the question at all. The exception is R. M. Ritter, The Oxford Style Manual (2003), which discusses the issue in at least two places. First, in the context of main-text occurrences (at 4.1.8 "Titles and subtitles of works"):
Foreign-language titles and subtitles follow the rules common to that language. ... Except in general for French and German titles, this consists for the most part of minimal capitalization: capitalizing only the first word in the title and subtitle, and any proper names, Transliterated or romanized titles follow this practice as well.
Second, in the context of translations of titles to be cited in a bibliography (at 13.11.1.11 "Bibliographies," under the subhead "Styles of translation"):
Titles of works cited must follow English rules: italics for published books and titles of journals, roman quoted [that is, regular nonitalic text placed within quotation marks] for articles in journals and unpublished dissertations. ... Capitalization, however, follows the rules for the language of the title.
As applied to your situation, Oxford seems to recommend translating the title Lorem ipsum into its English equivalent—presumably something like Greeking text—but retaining the original's style of lowercasing the second word in the title. Thus:
Greeking text is a pretty interesting book.
If that doesn't seem to indicate with adequate clarity the fact that Greeking text is a book title, you can make that fact somewhat clearer by including the original-language version of the title in brackets (though Oxford does not endorse this practice):
Greeking text [Lorem ipsum] is a pretty interesting book.
Best Answer
You do not need to capitalize nouns unless they're proper (or at the beginning of a sentence).
Particularly if your audience is made of English speakers who aren't expected to have knowledge of German vocabulary or grammar, the capitalization of an ordinary noun may cause confusion.
It's a good general practice to italicize foreign words that haven't become an accepted part of the English language yet, though.