To believe in Mahdi(A.S) and mahdaviat was not and is not an innovation and deviation but it is originated from hundreds or thousands of narrations of the last prophet(P.U.H),his followers ,his dependants and the sincere imams(A.S)
It is rooted from the original true references of Quran, and holy households (A.S). It has also been famous since the time of the last prophet (P.U.H).