Difference: Blameworthy and Praiseworthy
When it was said in the fifth verse (118) - ` had your Lord willed, He would have made all the people a single community' - the sense is that had it been the will of Allah, He would have made all human beings accept Islam by force. All of them would have then become nothing but Muslims without any difference remaining between them. But, such are the dictates of His wisdom that, in this world, Allah Ta` ala does not compel anyone to do something. Instead, He has entrusted man with a kind of choice under which he could do whatever good or bad he wishes to do. Then, human temperaments differ, ways differ and deeds differ. The outcome is that there always will be some people who would keep at loggerheads against the true faith - of course, with the exception of those whom Allah Ta` ala has blessed with His mercy, that is, those who have been following the noble prophets.
This tells us that ` difference' at this place means hostility and op-position to the true faith and the teachings of prophets. The difference based on Ijtihad, which is inevitable among religious authorities and jurists of Islam, an ongoing process since the period of the Sahabah, is not included under this purview, nor is it contrary to Divine mercy. In fact, it is the very dictate of Allah's wisdom and mercy. Those who have declared the differences among Mujtahid Imams to be counter to Mercy in terms of this verse, have done something which is itself counter to the context of this verse as well as counter to the consistent practice of the Sahabah and Tabi` in.
واللہُ سُبحَانہ و تعالیٰ اَعلم
And Allah is Pure and High and He knows everything best.