Of the aforementioned countries on this list, Pitcairn Island, with a population of 50, is the country with the highest percentage of self-proclaimed Christians, at 100%, and is tied with the Vatican in this respect.

Romania is the second highest on this list, at 98%

3rd on this list is Poland at 94.3 %

United States is 4th at 71% of its total population.

Looking at the actual numbers within a population, the United States has the greatest number compared to all countries, at approximately 230 million.

More Info: en.wikipedia.org