It's religious use has been so diluted by wide use by people of all faiths (or no faith) that it's simply a patriotic message, no more religious than "Support our County" or "United we Stand" or something along the likes of those.
How about "Famous Potatoes"? 
You know, that could be a real moneymaker. Have companies bid on having their slogans on a certain percentage of licence plates. The revenue could be used to replace the costs of vehicle registration and lower taxes.
(not saying I endorse such an idea, just a random thought to ponder).
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