[Surah al-A'rāf: 129],

And the Exalted said:

“Allah has promised those among you who believe and do righteous deeds that He will surely make them successors upon the earth, just as He made those before them successors.”

[Surah al-Nūr: 55]

And He said to the angels:

“I am appointing a successor upon the earth.”

[Surah al-Baqara: 30]

And He said:

“O David, We have made you a successor upon the earth.”

[Surah Sād: 26]

Most of these passages concern the latter becoming the successor of the former, even though the former did not appoint him as his successor.

A caliph is called a caliph because he succeeds the one before him, and Allah Most High caused him to succeed that predecessor, just as He caused night to succeed day and day to succeed night. What is meant is not that he is a successor to Allah, as some people have supposed, as we have explained at length elsewhere.

People call those who govern the Muslims’ affairs caliphs. The Prophet—may Allah bless him and grant him peace—said:

“You must adhere to my Sunnah and the Sunnah of the rightly guided, divinely directed caliphs after me.” (1)

It is well known that Uthmān did not appoint Ali as his successor, and Umar did not appoint any particular individual as his successor. He would say:

“If I appoint a successor, Abū Bakr appointed one; and if I do not appoint a successor, the Messenger of Allah—may Allah bless him and grant him peace—did not appoint one.”

Yet he would say to Abū Bakr, “O successor of the Messenger of Allah!”

The same applies to the Umayyad and Abbasid caliphs: many of them were not appointed by their predecessors. It is therefore clear that this designation applies generally to anyone who succeeds another.