I haven't actually had difficulty finding secular texts that inspire a sense of solemnity. If you want older language, which usually translates to a solemn tone, the St. Crispin's Day speech in Henry V is not religious in subject, but it is solemn. There is no lack of inspiring expressions whose subject is not religious.

Atheists don't often express themselves with the intent to inspire, but they do sometimes, and they can be fairly effective. I think Bertrand Russell did a good job, although he did write in prose. My point was that these expressions are either not absolute praise (i.e. science is a very good tool, but only a tool) or they're not really rational (i.e. "secular religion") or they're absolute praise of something almost too abstract to conceive (i.e. "Thank Goodness.")