..Yes, this is America and private companies can fire employees who represent themselves and the company that is objectionable to the powers that be..
Being a Christian is objectionable?
It is not objectionable to me, but I am not the person who owns the newspaper.
Moreover, it is not an issue of his being a Christian as there are Christians who are not opposed to gay marriage.
Thus, if the person had written an email that read, "I am a Christian and because of my faith believe that gays should be able to get married," I would hold the same opinion on this issue.
I am not all that sure that everyone here would hold that consistency on the issue.
And to restate my opinion, the owner of the newspaper can fire an employee for expressing a Christian view, either for or against gay marriage.