The Holy Quran clearly says "Let there be no compulsion in religion". In addition to this, Islam teaches that a person's faith must be pure and sincere, so it is certainly not something that can be forced on someone. In debunking the myth that Islam was "spread by the sword", the (non-Muslim) historian De Lacy O'Leary wrote:
"History makes it clear, however, that the legend of fanatical Muslims sweeping through the world and forcing Islam at the point of the sword upon conquered races is one of the most fantastically absurd myths that historians have ever accepted." (Islam at the Crossroads, London, 1923, p. 8.)
Temperamentally the Prophet Muhammad was not inclined to war; he had not once handled the sword in actual fighting up to the fifty-fifth year of his age, and this in a country where, owing to constant internecine warfare, fighting had become a vocation of the people. The religion which he preached, Islam (lit. peace or submission), was a religion of peace, laying stress on prayer to God and the service of humanity, and he was required to preach this religion; to deliver the message, not to enforce it on others:
There is no compulsion in religion. [2:256]
source : sibtayn
