Allah has commanded us to enjoin the good and forbid the evil, and this duty cannot be rendered without power and authority. Likewise, all other social duties such as jihād, maintaining peace and order, justice, organizing ḥajj and ‘īd festivals, redressing abuses, helping the oppressed, enforcing the ḥudūd, and so on, cannot be carried out without political power and authority.
Hudhayfah ibn al-Yaman said: ‘Islam is divided into eight parts. The prayer is one part, zakat is one part, fasting is one part, hajj is one part, ‘umrah is one part, jihad is one part, commanding what is known to be right is one part, and forbidding what is recognised as wrong is one part. Disappointed is he who has no part of Islam.’
From Muhammad, Messenger of Allah, to Jaifar and 'Abd, sons of al-Julanda, and may peace be upon those who follow Allah's guidance. To proceed, I invite you both to Islam; accept Islam and you will attain salvation, for I am Allah's Messenger to all mankind so that I may warn those who are alive and establish (Allah's) word over the unbelievers. If you declare your Islam I will appoint you both as governors [of your lands], but if you refuse then (know that) your dominion is only fleeting, and my horsemen will dismount in your courtyard, and my Prophethood will prevail over your dominion.
The Caliphate 