The One True Morty is a Morty who is seen as the god of Mortyism. He is taught about in a book called The Good Morty, and is portrayed as a god who will lead Mortys to stop following their Ricks and being independent, choosing the path of The One True Morty, leading them to a rewarding eternity in The Morty Afterlife Zone.
In Close Rick-Counters of the Rick Kind, a group of Mortys were being held in Evil Rick's torture chamber and had formed a cult, where they had read The Good Morty and believed that someday, The One True Morty will come and set them free of the torture chamber and relieve them of their sufferings. When Morty Smith from Dimension C-137 came, they believed him to be The One True Morty. Although that Morty was nothing like the one depicted in the book, and it would be unfair to say the he is The One True Morty, he followed through with exactly what the book said he would and freed all of the Mortys from their prison cells and took down Evil Rick's empire.