The line comes from a short poem by Abu al-'Ala' al-Ma'arri (d. 1057 CE):

في اللاذقية ضجة ما بين أحمد والــــمسيحُ
هذا بناقوس يدق وذا بمئذنة يــــــــــــصيحُ
كلٌ يعظِّم دنيه يا ليت شعري ما الصحيحُ ؟

The selection


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Note added at 1 day15 mins (2009-06-11 12:47:05 GMT)
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Sorry, I accidentally sent this before I finished typing. I thought having more of the Arabic context might help you. Anyway, the line:

This one here rings the church bell
While that one there shouts from the minaret;
[Each extols his own religion--]
I wish I knew the genuine truth!