Yavneh-YamAfter discussing the significance of the number eighteen regarding the prayer of the “Eighteen Benedictions,” the Bablyonian Talmud discusses the origin of what is considered to be the additional nineteenth blessing, denouncing non-believers, which has been incorporated into the prayer.

As to those eighteen benedictions — there are nineteen! R. Levi said : The benediction relating to the Minim was subsequently instituted at Jabneh…

Our Rabbis have taught: Simeon the Cotton-dealer arranged the eighteen benedictions in order in the presence of Rabban Gamaliel at Jabneh. Rabban Gamaliel asked the Sages, “Is there anyone who knows how to word the benediction relating to the Minim?” Samuel the Small stood up and worded it. In the following year he had forgotten it, and reflected for two or three hours ; but they did not discharge him.

Why did they not discharge him? For lo, Rab Judah said in the name of Rab: If [the Precentor] errs in all the [eighteen] benedictions we do not discharge him ; but [if he errs] in the benediction relating to the Minim we do discharge him, for fear that he is a Min!

It was different with Samuel the Small, because he had himself worded it. But there may be a fear that perhaps he had apostatized!  Abbai said : We have a tradition that a good man does not become bad. He does not? Behold it is written, “But when the righteous turneth away from his righteousness and committeth iniquity” (Ezek. xviii. 24)! That refers to a man who was originally wicked; but the man who is originally righteous does not. He does not? Behold there is a Mishnaic teaching: “Trust not in thyself until the day of thy death” ; and Johanan the High Priest held the Office of High Priest for eighty years and eventually became a Sadducee !

Abbai said: Johanan is the same person as Januaeus.

Raba said: They are different persons ; and Jannaeus was originally wicked, but Johanan originally righteous.
That is quite right according to Abbai ; but it is in contradiction with Raba’s view.

Raba can reply that even with one who was
originally righteous there is the possibility of his apostatizing.
If so, why did they not discharge [Samuel the Small]? It was different with him, because he had already commenced it. For Rab Judah in the name of Rab (another version: R. Joshua b. Levi) said: This teaching applies only to one who has not commenced it ; but if he had commenced it, he may finish it.