It's about as appropriate as "In Zeus We Trust". It's not something that is representative of the consensus of the American people.

They say statistics can be used to prove anything, so I'm not expecting this to change your mind, but poll after poll shows that the majority of people in the USA are Christians.

http://www.adherents.com/rel_USA.html

But there are two things: (1) relevance and (2) I'm guessing the consensus among Americans is that we wouldn't want merely a majority-rule democracy, but a democracy that also respects the rights of minorities.

Using the reasoning of popularity, we could put "Land of the Whites" on our money.

we (and when I say "we" I mean "you" because I'm not from the US) should be more concerned about the un-separation of church and state and less about the design

The large majority of Americans are Christian, and any even larger percentage believe in some sort of God. Do you really think a major hang-up preventing a separation of Church and State is the phrase on the money? This phrase is an indicator, not a contributing factor.

never said it was a factor