The Amidah's traditional Hebrew name, Shemoneh Esrei, means simply the eighteen, a name that has stuck even though the prayer recited today actually contains nineteen benedictions, a mismatch with a specific historical explanation: an additional benediction, Birkat HaMinim, directed against sectarians and informers, was added to the original set some time around the early rabbinic period, generally connected by scholars to the period of consolidation at Yavneh after the Temple's destruction, when the developing rabbinic movement was working to define its boundaries against groups it considered dangerously heterodox. Rather than renaming the prayer once a benediction had been added, the traditional name simply continued in use, so reciting the eighteen today means reciting nineteen, a small but telling piece of evidence for how liturgy can accumulate real changes while retaining an older name that no longer describes it precisely.
The scholarly question of exactly whom Birkat HaMinim originally targeted, and whether early Christian Jews specifically were among its intended targets, has been debated at length by historians of early Judaism and Christianity, since some early Christian sources describe experiencing exclusion connected to a curse recited in the synagogue around this period, while other scholars caution that the benediction's original wording and target are not preserved with enough precision in the earliest sources to settle definitively whether it was aimed specifically at Jewish Christians, at a broader range of sectarians, or at both without clear distinction.