The Freedom From Religion Foundation says the banner violates the Constitution because it was put up by members of the local fire department and hangs from two town-owned light posts over a county-owned road.
The banner was paid for by a Knights of Columbus group.
Officials say a similar banner has hung over the town about a dozen miles south of Philadelphia for the past 50 years.
Freedom From Religion says the banner should be moved to private property.
The mayor says he's asked a town lawyer to look into the situation.
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