Samuel Zwemer was called to preach the Gospel to Muslims, and he is without doubt the greatest missionary to the Islamic world. On June 28, 1890, he began his mission, sailing from the United States on a Dutch liner the Obdam. Zwemer stopped in Europe to meet with other evangelical groups and then traveled to Beirut. Zwemer traveled throughout much of the Muslim world proclaiming the Gospel of Jesus including the Balkans, India, China, Africa, the Middle East. On June 28th, 1933, he ventured as far as China to bring the Gospel to the Chinese Muslims.

Zwemer's greatest feat was to visit Sana'a in Yemen, a place no lone westerner had ever gone before. He had many adventures in Arabia. In one incident, his life was spared when a Bedouin guide swore a great oath that Zwemer was neither an Englishman nor a government agent. In another incident, a group of Arabs debated whether, or not, they should hold him for ransom. These adversities did not deter Zwemer as he distributed Gospel literature throughout the Muslim world.

Zwemer was dedicated to his mission of spreading the good news. He said : "No agency can penetrate Islam so deeply, abide so persistently, witness so daringly and influence so irresistibly as the printed page,". The only thing that stopped Zwemer from distributing Arabic leaflets and Bibles was confiscation. He dedicated his life to preaching the Gospel to the Muslims and the created an awareness of the spiritual needs of Muslims in the West.

Samuel Zwemer set up presses which printed many books - both to educate Westerners about the spiritual needs of Muslims, and Arabic language books to educate Muslims about Christ. He authored or co-authored at least 48 books in English.