As the Arab Empire declined why did Islam continue to spread?

One of the most significant events in history happened in the early seventh century in the deep interior of the Arabian Peninsula. The introduction of a new religion, Islam, to the world by the Prophet Mohammed united numerous warring Arab tribes. With their new found religious fervor, Arab armies march forth to spread the word of Islam. Arab invasions of surrounding lands resulted in the establishment of one of the largest empires in history, the Arab Empire.