The second caliph, Umar, continued the expansion of the Muslim state. He not only defeated the Persians but also secured Egypt from the Byzantines. Umars reign is one of the reasons the Muslim empire was so successful: he allowed people to retain their own religious beliefs. This greatly strengthened Islamic control as the increased freedom prompted not only happy citizens but also converts.
The caliphates position in religious affairs was strengthened when the third caliph, Uthman b. Affan, was selected to lead the Ottoman empire. He created a governmentally-sanctioned copy of the Quran. This action was incredibly beneficial to both political and religious organization, as it not only united the Islamic empire under one, clear version of the holy scripture, but it also showed the caliphate as capable and spiritually viable.
Last of the Four Righteous Caliphs Ali b. Abi Talib faced some of the most difficult challenges. Because he failed to punish Uthmans assassins in a timely manner, many of his former followers deserted him, and he was faced with civil war. He lost most control to Uthmans kinsman, Muawiya.
Ultimately, Uthman and Umar are the two more successful caliphs, as they expanded the empire dramatically, gaining new converts and willing citizens. On the other hand, Abu Bakr and Ali served necessary purposes: they dealt with their respective rebellions in a poised manner that ensured the continuation of Islamic rule. Without each, the Ottomans would have undoubtedly