Anshei Knesset HaGedolah instituted the prayers and the blessings (Berachot 33a). They established the wording of the Shemoneh Esrei and decided on the phrasing of all the berachot, including Birkot Keriat Shema and Birkot HaNehenin (blessings recited upon deriving pleasure from something). They also instituted the recital of the three daily prayers, Shacharit, Minchah, and Ma’ariv – Shacharit and Minchah as obligatory and Ma’ariv as optional.
The members of the court of Ezra HaSofer, which was established in the beginning of the time of the Second Temple, are called Anshei Knesset HaGedolah. This was the biggest court ever founded in Israel. It was comprised of 120 elders, among them prophets and sages, such as, Chagai, Zechariah, Malachi, Daniel, Chananyah, Misha’el, Azariah, Nechemiah the son of Chachaliah, Mordechai, Bilshan, and Zerubavel, as well as many other sages, the last one being Shimon HaTzaddik (Rambam’s introduction to Mishneh Torah).
During the time of the First Temple, Am Yisrael merited supreme spiritual accomplishments; the Shechinah dwelled in the Temple and the devoutly pious of Israel merited prophecy. Despite this, throughout most of the nation, grave sins including idol worship, forbidden relations and murder, were widespread and eventually caused the Temple to be destroyed and the nation of Israel to be exiled. Hence, when they were able to build the Second Temple, Anshei Knesset HaGedolah formed a supreme court, set boundaries to guard the laws of the Torah, instituted religious guidelines, and worded and arranged the prayers and berachot. They created a full framework for Jewish life which gave expression to the values of the Torah in an organized and established manner within everyday living, thereby distancing the nation from sin and bringing them closer to serving Hashem.
Even in the time of the First Temple the nation of Israel prayed to Hashem and thanked Him for all the good and blessing they received. However, that prayer did not have an organized wording. Since there was no exact text, the righteous and devout people would pray and recite berachot with great kavanah (intent), but the masses of the nation would exempt themselves with superficial prayers. Indeed, passionate prayer from the heart in one’s own words is the ideal method of prayer. Yet, in actuality, the routine concerns of everyday life wear us out, and without regular organized prayers, the public gradually drifts away from prayer services and eventually from Hashem. Following the establishment of the prayers and their fixed wording, all of Israel started to pray, and as a result, faith in Hashem intensified. This is what preserved the nation’s devotion, which remained a burning flame in the darkness of exile for two thousand years
Moreover, during the time of the First Temple, many people mistakenly thought that by offering sacrifices, their sins would be forgiven and they would merit Hashem’s blessing, even if they did not purify their hearts and correct their transgressions. The truth is that faith in Hashem, purification of the heart, and correcting one’s actions are of principal importance, as it says (Deuteronomy 10:12), “What does Hashem want of you? Only that you remain in awe of Hashem your God, so that you will follow all His paths and love Him, serving Hashem your God with all your heart and with all your soul.” The prophets severely condemned those who believed that the essence was to bring sacrifices without possessing and demonstrating true devotion to Hashem. As it is written (Isaiah 1:11-13), “‘Why do I need all your sacrifices?’ God asks. ‘I am sated with your burnt offerings of rams and the fat of fed beasts [brought without repentance or sincerity], and I have no desire for the blood of cattle, sheep, and goats. When you appear before Me, who asked you to do this, trampling My courts? Do not bring any more vain offerings; the incense of abomination they are to Me….’” By instituting the prayers, Anshei Knesset HaGedolah restored the proper order to serving Hashem, reminding us that faith, intent, and devotion are the basis of our lives, and they are expressed most appropriately via prayer, just as Rabbi Elazar said, “Prayer is greater than sacrifices” (Berachot 32b). With that in mind, we pray that we should be worthy of expressing our devotion to Hashem completely, both in prayer and in sacrifice.
. In Megillah 17b, it is told that Shimon Hapakuli arranged the eighteen berachot before Rabban Gamliel in the proper order, and a beraita is brought which clarifies the order of the berachot according to scriptural verses. A question is raised there, 18a: After Anshei Knesset HaGedolah instituted them, what was left for Shimon Hapakuli to arrange? The Gemara answers that they were forgotten and Shimon Hapakuli went back and rearranged them. One may ask, how can it be that the wording of the prayer that they were obligated to recite every day had been forgotten? The Shitah Mekubetzet resolves this question in Berachot 28b saying that they merely forgot the order of the berachot, which is what Shimon Hapakuli then restored. In the Rach’s and Me’iri’s version there is no mention in the Gemara that Shimon Hapakuli arranged anything in the Shemoneh Esrei, and hence there is no question or answer about that.
See further in this book 18:10, concerning the addition of the “LaMalshinim” berachah. Likewise, see further in this book chapter 2, note 1, in the words of the Mabit, the reason for the institution of the minyan by Anshei Knesset HaGedolah is that it replaced the Shechinah which was revealed through the sacrifices.