Spreading the Faith - A Look At Three Religions

Buddhism spread because a great king was converted.
Christianity spread because of a great missionary was converted.
Islam spread because of a great general was converted.

King Asoka
King Asoka (304–232 BC) was the third emperor of India's Mauryan dynasty and the first powerful monarch to embrace Buddhism. For the first two hundred years of Buddhism, missionary work were carried out Buddha's disciples dressed in yellow robes, wandering around the countryside. But the conversion of King Asoka changed all that. Asoka became a Buddhist after witnessing the horrific atrocities in his conquest of Kalinga. Thanks to the state resources that Asoka deployed, Buddhism was spread throughout the Mauryan Empire and beyond. He posted edicts engraved on iron pillars throughout his realm exhorting his people to lead an ethical life, and by following the principles himself, he inspired others to adopt Buddha’s teachings. He also sponsored missionaries who were sent to all the religions of India, Sri Lanka and Burma and to as far away as Libya, Egypt and Syria. If not for Asoka, Buddhism would remain a Indian small sect like Jainism.

Paul the Apostle
In Paul's own words on what he had done, "For I would not dare say anything except what Christ has accomplished through me to make the Gentiles obedient by word and deed, by the power of miraculous signs and wonders, and by the power of God's Spirit. As a result, I have fully proclaimed the good news about the Messiah from Jerusalem all the way around to Illyricum. (Romans 15:18-19 HCSB)"
Paul was on the way to persecute the Christians in Damascus when Jesus met him and converted him. Paul went on to become Christianity's greatest missionary, spreading the gospel within about 12 years to areas which is now Italy, Greece, the former Yugoslavia, Turkey, Syria and Israel. He did all these on foot and by sea, with no modern communication or transport technology, under persecution, without the patronage of political power or the might of an army.

Khalid ibn Al-Walid
Before his conversion to Islam, Khalid ibn Al-Walid (died 642AD) was renowned as being the only commander to have inflicted a serious battlefield defeat against Muhammad at the Battle of Uhud in 625AD. After his conversion, Khalid helped Muhammad to conquer Mecca and unite Arabia for Islam. After the death of Muhammad, he helped Caliph Abu Bakar (632-634AD) put down the apostasy and reunite Arabia for Islam. He conquered Mesopotamia (634AD) and Syria (637AD) and captured Jerusalem (637AD) under Caliph Umar (634-644AD).