You are correct. We are not supposed tell some one is kafir some one is moumin (believer). We dont know really who is who? no one is permitted to tell others as kafir. an outward looking moomin may be inwardly a kafir. How can we judge. Another person inadvertantly seen as a non believer may believe in one god then he is moumin.
so God only knows who is kafir who is moumin. No man has authority to call another man a kafir
