If the muslims all would stick to the quran and sunnah, without filtering out what they don't like, and with filtering out culture, than we can unite, but now, there are too many differences, there are so many muslims sticking to their culture, leaving things from the sunnah, trying to strive for dunya, while our goal should be akhira, if we all work for the same goal and follow the pure sunnah, then we can unite.