Christianity has spread because Christ died to His own desires and in the conclusion gave His life for us. Islam, on the other hand, has increased because Mohammed lived for his own desires and killed everyone who attempted to stand in his way.
We can see this very clearly in our generation, where countries are opening the door for the gospel of Jesus Christ. With the establishment of democracies, nations have the freedom to choose life or death, Bible or Koran, because God set before us life and death, blessing and curse (Deuteronomy 30:19). And we have to make our own choice, even if it is the wrong choice; this is freedom.
However, in every nation where the law of Islam's rules or even when most of the nation's people are Moslems, we see that there is no democracy, no human rights, no freedom to choose their own religion, because they say God has "perfected their religion for them, and has selected for them their religion Islam". A selection was made for them before they were born; this is slavery.