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Read the following Article in the official website:
“The Holy Quran promises that God will raise, among the Muslims, Khalifas (deputies or successors) to the Holy Prophet Muhammad:
"Allah has promised to those of you who believe, and do good, that He will surely make them khalifas in the earth as He made those before them to be khalifas " (24:55)
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By "those before them" are meant the Israelites. The Holy Prophet Muhammad said in explanation: "The Israelites used to be led by prophets; whenever a prophet died, another came after him. After me there is no prophet, but there will be khalifas and there will be many." (Bukhari, book 60, ch. 50)
The khalifas of the Holy Prophet include not only his successors who possessed worldly rule, such as Hazrat Abu Bakr and Umar, but also his spiritual successors who came to revive the true teachings of Islam in every age. The spiritual khalifas are also mentioned in another hadith of the Holy Prophet in the following words:
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In accordance with this tradition, Hazrat Mirza Ghulam Ahmad claimed to be the Mujaddid of the 14th century Hijra (1883-1979) and a spiritual khalifa of the Holy Prophet Muhammad. By believing him to be Mujaddid, we are not adding any new belief to Islam, but only following Islamic practice as recognised by the Ahl as-Sunna.
The text is available here.