Ki Tavo opens with two commandments that are fulfilled by way of a declaration –Bikkurim and Bi'ur Ma'asrot. These are just two of the commandments that require speech, all of which contain a declarative and demonstrative act of faith in God.
Both commandments require a fixed text to be recited out loud. The person who brings the Bikkurim must recite the 'Declaration of the Bikkurim', which contains a summary of the history of the Jewish People from the time of our forefathers till the entry into the Land of Israel. The Midrash on these verses forms the basis for the entire Pesach Haggadah. One who performs Bi'ur Ma'asrot, the second commandment in the section, is also required to recite a fixed text.
When the three-year cycle of Tithes (Trumot and Ma'asrot) is completed, a person must say the Declaration of the Tithes (Viduy Ma'asrot) in which he declares that he has completed all his obligations and asks God: "Look down from Your holy dwelling, from the heavens." (26:15)
Additional Torah obligations that require speech, reading or declaration include: Kriyat Shema, Birkhat HaTorah and Birkhat HaMazon. The sages of Anshei Knesset HaGedolah added to these and instituted the fixed text for our prayers and blessings that encompass the entire spectrum of Jewish life.
The obligation to pray and recite blessings is defined as 'service of the heart', which emphasizes both the ideas that are recited aloud and the thoughts we should be concentrating on when saying them. The siddur is therefore considered the fundamental book of Jewish Philosophy.
The principles of Jewish faith are defined not only in Maimonides' Thirteen Principles of Faith that, according to some customs, are recited daily, but also throughout every section of our prayers.
The set order of our prayers is not only meant to be a means of expressing a person's feelings towards his God, rather there is an additional external-declarative aspect. We learn this from the 'Declaration of the Bikkurim', in which the person who brings the First Fruit is required to say a short declaration praising God for bringing the children of Israel to the Land flowing with milk and honey.
When a group of people gathers for prayers, they are partaking in a revolutionary 'demonstration'. Outwardly, they are demonstrating their faith in God with their prayers and inwardly they form a cohesive group with the same sense of purpose, enhancing the sense of belonging and further boosting the internal fortitude of each one of the members.