Tashlich emerged during the “Amoraic Period” (A.D. 200-600). During that era, there was a mental shift away from worship in the Temple in Jerusalem (and its necessity) to alternate means of finding communion with God. Later Rabbinic authorities would expand on these ideas to claim that sacrifices were never essential. These developments are particularly relevant to Rosh Hashanah and Yom Kippur since the themes of substitutionary sacrifice and righteousness run through the heart of those holy days. Consequently, in Jewish thought, God accepts our prayers and repentance by virtue of the patriarchs’ faithful obedience unto death, namely Abraham and Isaac.
The practice of Tashlich, in general, is rooted in an early Midrash (note this scholarly explanation of “midrash”: “Midrash, aside from being a method of interpretation, is also used to designate the literature that arose from such interpretation”). In this Midrash, as Abraham and Isaac made their way to Moriah to offer the sacrifice, Satan approached them to tempt Isaac,
“[Satan] Said to him, ‘You poor wretch son of a poor wretch of a woman, how many fasts did your mother fast so that you could be born, and now this old fool is on his way to sacrifice you.’ [Isaac] replied, ‘Even if it is so, I will not submit to my evil inclination, and I will not transgress the will of my father.’
The Midrash continues to state that once Satan saw that he could dissuade neither Isaac nor Abraham through reasoning, he trapped them in a flash flood at a river they had to cross. So, Abraham and Isaac called out to God by the river, and it dried up so that they could proceed on their way. The Rabbis teach, “It is on account of this that we walk to a river to remember the binding of Isaac, to awaken mercy upon their children as they surrendered themselves and withstood such a great trial.” Interestingly, with the exclusion of a single exceptionally long variant of the Tashlich service written by Rabbi Yosef Haim of Baghdad, none of the liturgies contain any direct reference to the binding of Isaac.
Throughout the centuries, Christians have understood Isaac to be a “type and shadow” of the Messiah due, in no small part, to the Apostle (and Rabbi) Paul’s writings in Galatians. The Midrash mentioned above has echoes of passages from the New Testament:
“He humbled Himself— becoming obedient to the point of death, even death on a cross” (Philippians 2:8), and
“He was saying, ‘Abba, Father, all things are possible for You! Take this cup from Me! Yet not what I will, but what You will” (Mark 14:36)