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.