The Muslims do not pin their hopes on someone who will save or deliver them, as is the case with the Jews, the Christians and the Raafidis (Shi’ah).
Islam is complete and its rulings are complete. Perhaps what is meant in the question is the Mahdi. He will be one of the reformers of this ummah through whom Allaah will fill the earth with justice and fairness after it was filled with injustice and unfairness. Like other reformers, he will follow the Qur’aan and Sunnah and will rule according to them.
Some people misunderstand the ahaadeeth about the Mahdi and the descent of Eesa ibn Maryam (Son of Mary), so they do not work for Islam or to call people to Allaah because they are waiting for the Messiah to come down or for the Mahdi to appear. This is undoubtedly a mistake.
The Muslims have to strive to make the word of Allaah supreme, to make His religion prevail and to convey the message of truth to all people.
We can imagine how much the Muslims would have declined and how weak they would have become if the earlier Muslims had understood this matter in this mistaken manner and acted accordingly.
Would they have been able to defeat the Tatars and Crusaders, and conquer other lands?