When he comes, Imaam Mahdi will not be able to do More than what the Prophet صلى الله عليه وسلم did by Ahmed Abu Turaab

Shaikh al-Albaani said, “O my brother Muslim, know that many of the Muslims today have strayed from the truth in this issue. From them are those who hold it to be a settled fact that an Islamic state will not be established except with the emergence of the Mahdi! And this is a myth and is misguidance which the devil throws into the hearts of many of the masses, especially the Sufis among them–and there is nothing in the hadiths of the Mahdi which indicates that at all.

Rather, all of those hadiths do not go beyond the fact that the Prophet صلى الله عليه وسلم gave glad tidings to the Muslims of [the coming of] a man from his household, and he described him with outstanding characteristics, the most important of them being the fact that he will judge by Islaam and spread justice amongst mankind.

So in reality, he is one of the revivers which Allaah sends at the head of every one hundred years, as is authentically reported from him صلى الله عليه وسلم. So just as that [i.e., the emergence of a reviver at the head of every one hundred years] does not necessitate the abandonment of striving to seek knowledge and acting upon it to revive the religion, then likewise, the emergence of the Mahdi does not mean relying totally on him [tawaakul] without making preparations and without taking steps to establish Allaah’s rule on earth.

Rather the opposite of that is correct. For indeed the Mahdi’s efforts will not be greater than those of our Prophet Muhammad صلى الله عليه وسلم who spent twenty-three years working to establish the foundations of Islaam and its state–so what can the Mahdi possible do if he emerged today and found the Muslims split into sects and groups, and [found] their [bad] scholars, except for a few of [the good ones among] them, to have been taken as heads by the people! He would not be able to establish the nation state of Islaam except after uniting their word and uniting their ranks, under one banner, and this without doubt requires a long time which Allaah knows best as to just how long.

So both the Legislation and the intellect demand that the sincere Muslims carry out this obligation such that when the Mahdi does come, he will not be required except to lead them to victory and if he doesn’t come [in their time, then at the very least] they will have fulfilled what was obligatory upon them, and Allaah says, “And say, ‘Do [as you will], for Allaah will see your deeds, and [so will] His Messenger …’” [Tawbah 9:105]

As-Saheehah, 4/42-43.