Islam is the only religion which addresses all human beings and deals with all of them with equality with out any discrimination. Islam states the fact: All human beings are from one father (Adam) and one mother (Hawwa).
Political reasons caused depriving many people from knowing Islam. When people are able to know Islam, millions will embrace it because it has The Real Evidence from God which can never be found in any other faith or ideology.
Number of followers is never the only evidence of the reality of any religion or idea because we need to study the political reasons. Intellectually, Islam is the only religion which has the rational answers for the questions of those who think properly.
Scholars from all over the world admit the superiority of Islam which has has resulted despite all the obstacles, in making Islam the most spreading religion in the world today. https://www.bbc.co.uk/news/av/world-39279631