I thought this was a good letter. The author acknowledged each side of the issue. Because the writings of the Founders are varied and open to interpretation, the author chooses to move to a different proposal: We need a nation that allows freedom of religion and freedom from siding with any particular religion.
Owens then goes on to note what happens in many countries where there is no separation of church and state. It's always surprised me that those who want a nation founded on one religion to be a nation of that religion, ignore the wide variety of religions we have, including the various takes on Christian religion. 
Does this nation need a religion sanctioned by the government? Some people believe this is the only way to remain protected by the God of that religion. Ironically, adopting that religion means accepting this view of God that would forsake his followers for no longer being followers. To me, that just seems vengeful and based on superstition.