Why announce that I'm about to praise God? Why not just start?
Fair question. This line praises nothing. It only says you are about to. Of the sources we could open, only Rav Kook comments on this line directly. The others write about the moment before prayer, and about similar lines.
The Mishnah says a person should not stand up to pray except from a serious frame of mind. It adds that the early pious ones would wait an hour first, so they could aim their hearts at God. The Rambam (Maimonides, died 1204) turns this into a rule: clear your mind of other thoughts, picture yourself standing before God's Presence, and sit a little before praying in order to do it.
Few of us wait an hour. Nothing in these sources says this line was written to replace that wait. But it gives you a place to take a short one: you stop and say in words what you are about to do.
Rav Kook starts with the word "mouth." Speech has two sources, he writes. Ordinary talk comes from the pictures the world prints on your mind. Holy speech comes from the source of all knowledge. So a person about to begin praying must give his mouth an invitation fitting for holiness, so that his words flow from the holy source.
Rabbi Natan of Breslov writes about a similar line, "I am ready and prepared," said before mitzvot. We cannot do mitzvot properly by our own strength, he says. What we can do is say aloud that we want to. That wanting is the main thing, and then, with God's help, the mitzvah gets done.
Not every rabbi liked adding statements before a mitzvah. The Tallit prayer tells how one leading rabbi argued that a mitzvah with a blessing needs only its blessing. This line is a later custom, not a requirement from the Gemara.
Mishnah Berakhot 5:1,Rambam, Laws of Prayer 4:16,Rav Kook, Olat Re'iyah, on Hareini Mezamen,Likutei Halakhot, Grace After Meals 4:21,Noda BiYehuda, first edition, Yoreh Deah 93It says it unites God and His Presence. Isn't God already one?
The sources know this problem. The Shema says Hashem is one. The Rambam writes that God is one in a way nothing else is: not a group of parts, and not a body that can be divided. If so, what is there to unite?
Start with the word Shekhinah. It comes from the root "to dwell." In the Torah, God says, "Let them make Me a sanctuary, and I will dwell among them." In the Gemara, the Presence means God close by, in the world.
The Sages picture that Presence as sharing Israel's story. Rabbi Shimon bar Yochai, a sage of the Mishnah, says that wherever Israel was exiled, the Presence went with them, and in the redemption God will return with them. In the Mishnah, Rabbi Meir says that when a person suffers, the Presence says, "My head is heavy, my arm is heavy," so to speak, and he concludes that God hurts even over the blood of the wicked, and all the more over the righteous.
The kabbalists take that picture further. Rabbi Elazar Rokeach (a kabbalist and rabbi of Amsterdam, whose commentary on the Mishnah was printed there in 1740) writes that in exile there is a separation between the Holy One and His Presence, and only at certain times is there a union. So every mitzvah, and all Torah study, should aim to unite them, using the words of our line: "through the hidden and concealed one."
Rav Kook describes it differently. For him the union is between holiness that flows from a source without limits and holiness that marks limits (times and places, actions and thoughts, wishes) inside which the divine light moves.
So the Rambam says God cannot be split, and the siddur speaks of God and His Presence being joined. The sources we could open do not stop to reconcile the two in so many words. What they give is the picture: a Presence close enough to share the exile, and a prayer that asks for the distance to close. The question stays partly open.
Mishneh Torah, Foundations of the Torah 1:7,Shemot 25:8,Megillah 29a,Mishnah Sanhedrin 6:5,Maaseh Rokeach on Mishnah, Nazir 29,Rav Kook, Olat Re'iyah, on Leshem YichudI'm saying this alone and half awake. Who is "all Israel"?
The last words are "in the name of all Israel." You are not saying it for yourself alone. You are saying it for the whole Jewish people, even if you are alone in a room.
Jewish law cares about this. The Rambam writes that the prayer of the community is always heard, even if there are sinners in it, because God does not despise the prayer of the many. So a person should join himself to the community, and not pray alone when he can pray with it. In Pirkei Avot, Hillel says: do not separate yourself from the community.
These words go back at least to the Arizal. When Rabbi Chaim Vital wrote down an instruction of his teacher, the Arizal (Rabbi Yitzchak Luria, the kabbalist of Tzfat who died in 1572), to say a statement of intent before doing a mitzvah or giving charity, "in the name of all Israel" was already part of it. The Tallit prayer tells that story.
Rav Kook adds a thought about the whole line, the joining of God and His Presence through the hidden one. He writes that not every individual can be gripped by this lofty idea, only the nation as a whole.
The Hareini Mekabel prayer tells a related instruction from the Arizal: before praying, decide to love every Jew as yourself.
None of these sources promises you will feel it at that hour. They describe a choice about who you are speaking for, made in words.
Mishneh Torah, Laws of Prayer 8:1,Pirkei Avot 2:4,Rabbi Chaim Vital, Sha'ar Ruach HaKodesh 16:3,Rav Kook, Olat Re'iyah, on Leshem Yichud