The sacred existence of Imam Mahdi (a.s.) is a continuation of the prophets’ movements and divine calls
The sacred existence of Imam Mahdi (a.s.) is a continuation of the movement of divine prophets and their calls, since the beginning of the history till today. As you read in Nodbeh supplication, there is a related and attached sequence in the history of mankind since the time of Adam till the last prophet-- prophet Muhammad (pbuh)-- followed by his successors and his progeny, reaching Imam Mahdi. This means the great movement of the prophets and the divine calls by the prophets have never stopped throughout history. Humanity has always needed a prophet, a divine call, and the individuals who call the people to worship God. And this need persists today. As the time has passed, humanity has become closer to the prophets' teachings. Today, with advances in intellectuality, civilization and wisdom, human societies have come to comprehend many of the teachings of the prophets which were not understandable for them tens of centuries ago. The topics of justice, freedom, human dignity, etc. -- which are popular in the world today-- were originally raised by the prophets. At the time of the prophets, the masses of the people could not comprehend these concepts. The fact that the prophets were sent consecutively and their message spread, has made these thoughts inherent in the people's minds, hearts and nature-- generation after generation. Those messengers of Allah have not stopped their succession: Imam Mahdi (a.s.) assures the succession of the honorable messengers of Allah.
That is, you can see the call by Abraham, Moses, Jesus, the call by all the prophets and redeemers sent by God and the call by prophet Muhammad (pbuh) represented in Imam Mahdi (a.s.). His excellency is the heir of all of them and holds their flag, inviting the world to the same teachings that the prophets have brought over time and have offered to humanity. This is an important point.