The Council of Jamnia, presumably held in Yavneh in the Holy Land, was a hypothetical late 1st-century council... which may... have been the occasion when the Jewish authorities decided to exclude believers in Jesus as the Messiah from synagogue attendance... The writing of the Birkat ha-Minim benediction is attributed to Shmuel ha-Katan at the supposed Council of Jamnia.
Modern scholarship has generally evaluated that the Birkat haMinim probably did originally include Jewish Christians before Christianity became markedly a gentile religion. It is the 12th of the Eighteen Benedictions or Amidah.
According to the Babylonian Talmud Tractate Berakhot 28b–29a, Shmuel ha-Katan was responsible for the writing of the Birkat haMinim:
"Rabban Gamaliel said to the sages: Is there no one who knows how to compose a benediction against the minim? Samuel Ha-Qatan stood up and composed it."
The blessing exists in various forms. Two medieval Cairo Genizah copies include references to both minim and Notzrim ("Nazarenes", i.e. "Christians").
"For the apostates let there be no hope. And let the arrogant government be speedily uprooted in our days. Let the noẓerim and the minim be destroyed in a moment. And let them be blotted out of the Book of Life and not be inscribed together with the righteous. Blessed art thou, O Lord, who humblest the arrogant" (Schechter)."
The extent of reference to Notzrim, or application of minim to Christians is debated. In his analysis of various scholarly views on the Birkat haMinim, Pieter W. van der Horst sums up,
"It is certain that minim in Tannaitic times are always Jews... It is certain that notsrim was not a part of the earliest version(s) of our berakhah."
During the medieval period, whether the birkat included Christians or not was the subject of disputations, a potential cause for persecution and thus a matter relevant for the safety of Jewish communities. It is generally viewed in modern studies that the term "heretics" at an early point in split of early Christianity and Judaism had included Jewish Christians. It was David Flusser's view (1992) that the Birkat haMinim was added in reference to the Sadducees.
Many scholars have seen reference to the Birkat haMinim in Justin Martyr's complaint to Trypho of the Jews "cursing in your synagogues those that believe on Christ." Reuven Kimelman (1981) challenged this, noting that Justin's description places the curse in the wrong sequence in the synagogue service.
7. Rashi on BT Megillah 17b: "The minim are disciples of Jesus the Notzri which is why they put Birkat haMinim ..."
8. Marvin R. Wilson, Our father Abraham: Jewish roots of the Christian Faith, Wm. B. Eerdmans: 1989, p.68 "We must emphasize that only two texts of the Birkat ha-Minim (both found in the Cairo Genizah) explicitly mention Christians. Both texts refer to "the Christians [notzrim, ie, the Nazarenes] and the heretics / minim]. "
9. William David Davies, Louis Finkelstein, Steven T. Katz (eds.) The Cambridge History of Judaism: The late Roman-Rabbinic period 2006, p.291 "He (Gedaliah Alon) proposes that the original Yavnean version of the Birkat ha-Minim, following the medieval Genizah fragment, included both minim and 'Nazarenes,' and that 'in this liturgical fragment minim and Notzrim are synonymous, ie, that both refer to the Jewish Christians.' But Alon's 'assumption' about the form of the original version is unconvincing, and this not least because, if the terms minim and Notzrim are synonymous, there would be no need for both of them in the benediction. Thus, as already argued, it appears more reasonable to suspect that Notzrim was added to a pre-existing malediction after the period of Yavneh – and most likely after the Bar Kochba Revolt (or later)"
I think Justin Martyr's (100-- 155 AD) reference to cursing Christians in the synagogues sounds relevant to the question of the Birkat Ha-Minim's original form or direction, especially if the "Birkat" was composed during the first c. AD when divisions between Jewish Christians and the rabbis was forming.