After 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!
Abbai said: Johanan is the same person as Januaeus.
Raba can reply that even with one who was originally righteous there is the possibility of his apostatizing.