In recent weeks at work, I received two letters in which the authors used "yours in Christ" before their signature in place of the commonly accepted "sincerely," "regards," etc. These were both business-related letters and not personal correspondence. One of them, as it so happens, had even been notarized.
When I was learning professional correspondence in high school and reviewing these lessons in college, I don't ever remember hearing that "yours in Christ" was an acceptable way to end a business letter. I went back and checked some style manuals to be sure, and my memory appears to be correct. So, when did this become acceptable? I should probably mention that both of the letters I received were written by college graduates. It seems fairly safe to assume that both would have had at least some training in professional correspondence during their education.