God (Allah)
The main concept of Islam is tawhid, the oneness of god. Islam is a monotheistic religion which means that they believe in one god, Allah which Muslims believe he/she's capable of doing anything. Since no one has ever seen or met Allah except Prophet Muhammad, the gender, physical features, voice and other characteristics of Allah is uncertain and unclear.
Prophet
Muhammad - Muhammad born in Mecca in about 570 A.D. Muslims believe that he receive announcements from Allah and work as the bond between Muslims and Allah. One of his greatest achievement was the Quran, the holy book of Islam. Muhammad is the messenger of the god. (To see more information of Muhammad go to http://gotomecca.weebly.com/prophet-muhammad.html )
Caliphs
Muslims know the first four caliphs as the Rashidun (rightly guided)
Abu Bakr - Abu Bakr was chosen to be the leader because he was a close friend of Muhammad as well as one of the first converts to Islam. Abu Bakr faced problems with some leaders of Arab tribes but eventually was able to reunite the tribes and convert them all under Islam. Abu Bakr was kwon as a caliph which is a successor to Muhammad. Under the first four caliphs (Bakr and other caliphs after), the Muslims united the Arabs and conquered large portions of the Byzantine empire and the Middle East. They defeated Persians and took Palestine from Byzantine (including Damascus, Syria and Jerusalem).
Umar ibn al-Khattab - Umar ibn al-Khattab was the second caliph of the Islamic empire who was a member of Muhammad's Quraysh tribe. It is ironic that at first, Umar actually was in opposition to the new religion which was based on the life and teachings of Muhammad. However, Umar eventually converted to Islam and became one of the strongest and spirited supporters and advisers of Islam and Muhammad. Muslims believe that the three verses of the Qur'an, the holy book of Islam were divulged due to Umar's request. Umar was a very clever ad intelligent leader who was capable of achieving many entity that helped the empire and Islam grow. He extended the territory from Syria all the way to Egypt, extending eastward which now is Iraq and southwestern Iran. Also, the state was able to grow rapidly and become an empire because Umar created the social and political structure. Although he was able to do such things, only the Sunni Muslims consider Umar as an admirable Calph since the Shi'ite Muslims believe that Umar is not acceptable as a leader because he is not directly (family) related to Muhammad. In addition, Umar married Muhammad's daughter Hafsah.
Uthmān ibn Affān - Uthmān ibn Affān was the third of the major caliphs of the islamic empire who was actually distantly related to Muhammad; he was a member if the Umayyad family. Uthmān was actually one of the first people to convert and join Muhammad and other Muslims. Although he was good looking, elegant, and generous the empire did not last peaceful throughout his entire 12 years in power. During his first 6 years, he managed to train his armies and raid lands in eastern Mediterranean, including Cyprus and Rhodes. Also he made an effort to unify the empire since it has such an diverse population. However, during his other 6 years in authority, people started to rebel due to lack of control and weak government. Many people were perturbed at the fact that he assigned his own family who lacked skills and abilities to be the governors. In addition, he married two of Muhammad's daughters- first Ruqayyah and after her death, Umm Kulthum.
Alī ibn Abī Tālib - Alī ibn Abī Tālib was the fourth caliph of the Islamic Empire who is a cousin and son-in-law of the Prophet Muhammad and was therefore one of the earliest people to convert to Islam. He was that was primarily supported by the Shi'ite Muslims who regarded that one of Muhammad's family becoming the leader was the right thing to do. They believed that Muhammad had specifically chosen Alī as his immediate successor. However, the Sunni Muslims strongly disagrees with them and claims that any male Muslim who has the abilities to become a leader should be chosen as the leader. Alī had an magnificant understanding of the Qur’ān and the Sunnah of the Prophet. Muslims surmised that the Qur’ān contains God’s actual words which is only to be revealed to the Prophet and recorded by his followers, including Alī. Alī was clever in that he advised the first three caliphs—Abū Bakr, Umar ibn al-Khattāb, and Uthmān ibn Affān. However he opposed some of their governing methods which led to disagreements. Ali was also a shrewd military commander since he defeated many forces and armies. For instance, the battle with the Mu`āwiyah's at Siffin where he used peace and rightfulness to settle the battle and the conflict with no harm after seeing his people suffer. Alī agreed to the use of arbitrators which appointed representatives given the power to settle the conflict peacefully. However, Mu`āwiyah seeked revenge and eventually he and his troops made raids into Iraq, Egypt, and Arabia. In addition, he married Muhammad's daughter Fātimah.
Abu Bakr - Abu Bakr was chosen to be the leader because he was a close friend of Muhammad as well as one of the first converts to Islam. Abu Bakr faced problems with some leaders of Arab tribes but eventually was able to reunite the tribes and convert them all under Islam. Abu Bakr was kwon as a caliph which is a successor to Muhammad. Under the first four caliphs (Bakr and other caliphs after), the Muslims united the Arabs and conquered large portions of the Byzantine empire and the Middle East. They defeated Persians and took Palestine from Byzantine (including Damascus, Syria and Jerusalem).
Umar ibn al-Khattab - Umar ibn al-Khattab was the second caliph of the Islamic empire who was a member of Muhammad's Quraysh tribe. It is ironic that at first, Umar actually was in opposition to the new religion which was based on the life and teachings of Muhammad. However, Umar eventually converted to Islam and became one of the strongest and spirited supporters and advisers of Islam and Muhammad. Muslims believe that the three verses of the Qur'an, the holy book of Islam were divulged due to Umar's request. Umar was a very clever ad intelligent leader who was capable of achieving many entity that helped the empire and Islam grow. He extended the territory from Syria all the way to Egypt, extending eastward which now is Iraq and southwestern Iran. Also, the state was able to grow rapidly and become an empire because Umar created the social and political structure. Although he was able to do such things, only the Sunni Muslims consider Umar as an admirable Calph since the Shi'ite Muslims believe that Umar is not acceptable as a leader because he is not directly (family) related to Muhammad. In addition, Umar married Muhammad's daughter Hafsah.
Uthmān ibn Affān - Uthmān ibn Affān was the third of the major caliphs of the islamic empire who was actually distantly related to Muhammad; he was a member if the Umayyad family. Uthmān was actually one of the first people to convert and join Muhammad and other Muslims. Although he was good looking, elegant, and generous the empire did not last peaceful throughout his entire 12 years in power. During his first 6 years, he managed to train his armies and raid lands in eastern Mediterranean, including Cyprus and Rhodes. Also he made an effort to unify the empire since it has such an diverse population. However, during his other 6 years in authority, people started to rebel due to lack of control and weak government. Many people were perturbed at the fact that he assigned his own family who lacked skills and abilities to be the governors. In addition, he married two of Muhammad's daughters- first Ruqayyah and after her death, Umm Kulthum.
Alī ibn Abī Tālib - Alī ibn Abī Tālib was the fourth caliph of the Islamic Empire who is a cousin and son-in-law of the Prophet Muhammad and was therefore one of the earliest people to convert to Islam. He was that was primarily supported by the Shi'ite Muslims who regarded that one of Muhammad's family becoming the leader was the right thing to do. They believed that Muhammad had specifically chosen Alī as his immediate successor. However, the Sunni Muslims strongly disagrees with them and claims that any male Muslim who has the abilities to become a leader should be chosen as the leader. Alī had an magnificant understanding of the Qur’ān and the Sunnah of the Prophet. Muslims surmised that the Qur’ān contains God’s actual words which is only to be revealed to the Prophet and recorded by his followers, including Alī. Alī was clever in that he advised the first three caliphs—Abū Bakr, Umar ibn al-Khattāb, and Uthmān ibn Affān. However he opposed some of their governing methods which led to disagreements. Ali was also a shrewd military commander since he defeated many forces and armies. For instance, the battle with the Mu`āwiyah's at Siffin where he used peace and rightfulness to settle the battle and the conflict with no harm after seeing his people suffer. Alī agreed to the use of arbitrators which appointed representatives given the power to settle the conflict peacefully. However, Mu`āwiyah seeked revenge and eventually he and his troops made raids into Iraq, Egypt, and Arabia. In addition, he married Muhammad's daughter Fātimah.