Prophet Muhammad PBUH in his last address has mentioned.
"Learn that every Muslim is a brother to every Muslim and that the Muslims constitute one brotherhood."
Here in the forum, there is a thread going on that is questioning does Ummah Exist and many seniors members (and supposedly the elders of this society) are mentioning that it does not. When it has been told that a Muslim is brother of every Muslim and that the Muslims Constitute one Brotherhood, what is matter of opinion in it anymore? If one is lose at his faith, he can refuse to accept sayings of Quran or Sunnah but if you say La Ialaha Illala, Muhammad Rasool Allah then its not a matter of opinion anymore.
Muslims Constitute one Brotherhood and that brotherhood exists even if you fail to see as this is not made by political or regional interests, it has been made by Prophet Muhammad and it exists. If you find this brotherhood is not as strong as it should be, may be you should equally question if we are as good Muslims as we should be.
I am purposefully keeping this threat out of the thread that is spreading doubts and asking people about their opinion weather Ummah exists or not. In faith, there is no opinion. You take it and become Muslim or you leave it and it doesn't matter what you become after-words.