Hazrat Moulana Ashraf Ali Thanwi (rahmatullahi ‘alaih) once mentioned:

Kufr is the root cause of all contemptible traits and evil qualities. On the other hand, Islam is the root cause of all praiseworthy qualities and good actions. When kufr is the source of all evil and Islam is the source of all good, it is extremely strange that at times unity is seen among the disbelievers while disunity is found among the Believers. One should realise that Islam is the basis of unity and kufr is the basis of disunity, and the apparent imbalance in the condition of unity among the kuffaar and disunity among the believers is on account of some external factor or personal motives. (Malfoozaat Hakeemul Ummat 7/194)

Source: Ihyaauddeen.co.za