According to hadiths, after the return of Imam Mahdi (aj), Prophet Isa (pbuh) will descend onto the earth by the will of Allah (swt), and will pray behind the imam (aj) as one of his companions in everyone’s presence. The prophet of Islam (pbuh) has been narrated saying: “By the one who sent me as the true giver of glad tidings, if even only one day remains for the life of this world, Allah (swt) will make that day so long so that my son, [Imam] Mahdi will emerge; Isa, “the spirit of God” will reveal and pray behind him, the world then will shine with the light of its Lord, and the government of [Imam] Mahdi will stretch from the East to the West.”
Since many are followers of Prophet Isa (pbuh) and believe him to be the savior and peacemaker of the world, when they witness such a scene, they will become followers of Imam Mahdi (aj) and believe in him. This matter can be considered one of the things that will factor into his victory and universal rise.
For further information, see:
1- The signs of after Imam Mahdi’s (aj) rise, Question 3006 (website: 3578).