Why has there been a gap in the spread of Islam after the 1st century?
Actually, there has not been a gap that remains until our time. There have been many changes in leadership. When the Abbasids came into power, their empire spread at a massive rateand then declined. In the Ottoman period it also started spreading again, and then declined.
If we look over the scale of history in the last 100 years, it is not a very big gap; rather it just seems to be. If we look at nations 1400 years ago, what nations existed at that time, exist today as well, and only Islam existed, in spite of the political divisions that took place. The rate of growth of Islam internationally far exceeded Christianity, and this has continued up until this time. Jesus spread Islam as well, and now the number of Muslims is only a few hundred million less than Christians. Muslims are expected to catch up, In sha Allah. When the Muslim nation has vitality, people revive the teachings, teachings have an impact on the societies as a whole, and the teachings are conveyed, Islam will spread. If we have missionary activities, Islam will spread far and wide. We don't have missionary traditions in the sense that people are trained (i.e. religious people) whose responsibility is mainly to convey the religion.
 Imaam Ibn Kathir, Al-Bidayah waNihayah, Vol 11, p 143-715
 Imaam Ibn Kathir, Al-Bidayah waNihayah, Vol 13, p 17< Back to Questions