O mankind! We have created you from a male and a female, and made you into nations and tribes, that you may know one another. Verily, the most honourable of you with Allah is that (believer) who has At-Taqwa [i.e. one of the Muttaqun (pious - see V.2:2). ...
Please see 30:22 and 10:19 as well.
Muslims in the real world are (semi) united in theory only when faced with an external enemy. The reality on the ground is so different. Muslims are killing more muslims in the name of Allah. Where is the unity? Ethnic and sectarian differences rule the day.
We, therefore, cannot ignore ethnic and sectatian realities staring us in the face each day. At the end of the day, a Morrocan tends to gravitate to those from his own country (whether in Islamabad of Dhaka) and the Pakistani tends to gravitate towards their won in The Gulf states.
The message should be unity in diversity based on a common spiritual bond, rather than rejecting ethnic and tribal affiliations as anti-god and anti-islamic.