The midrash (Bamidbar Rabba 15:18) connects the pasuk "Assemble for me seventy men" (Bamidbar 11:16) to the pasuk in Amos (9:6): "He builds upper chambers in the heavens, and His aguda (binding together) He founded on the earth." The midrash takes this to mean that, kav’yachol, Hashem’s throne in the heavens is only firm if Israel is bound together in unity. Another powerful midrash along this line says that even when the Northern Kingdom of Israel was guilty of idol worship, Hashem left them intact because they were connected to each other (Bereishit Rabba 38:6). What is so positive about the unity between sinners?
On the pasuk describing Bnei Yisrael’s preparations to receive the Torah, "They stood at the bottom of the mountain" (Shemot 19:17), Chazal say that Hashem held the mountain over them to make sure they accept it (Shabbat 88a). A midrash (Shemot Rabba 42:8) says that Bnei Yisrael’s statement "We shall do and hear" lacked full conviction. How could that be considering that Bnei Yisrael were so praised and rewarded for these words (see gemara ibid.).
Bnei Yisrael made it to the point of accepting the Torah by jumping through a great number of levels from the bottom spiritual rung (49th level of impurity) to the highest levels. How did this happen? They were aided by miracles and revelations, in line with Chazal’s comment that maidservants saw more divinity at the splitting of the sea than Yechezkel saw in his prophecies (Mechilta, Beshalach 3). These revelations left no room for doubt about Hashem, and when there is no doubt, what choice does one have but to accept the Torah that Hashem is giving you? This is the holding of the mountain over their head. While Bnei Yisrael did not mean "We shall do and hear" insincerely, still it was the result of a rare level of amazement. Since the commitment they naturally made did not have a chance to penetrate their consciousness, Chazal viewed it as equivalent to an incomplete acceptance.
But still how did they make it to this exalted level? It is by encamping at Sinai in a manner of unity that made them fit to be described in the singular (see Rashi, Shemot 19:2). The logic is as follows. Every Jew has two special powers: the innate character of greatness (segula); the power to act properly. That which we say, "Even though a Jew has sinned, he is still a Jew" (Sanhedrin 44a) emanates from the power of segula. The national power of segula is linked to the unity within the nation, making them a distinct nation. Then, the combination of the segulot of each part of the nation enables the practical power of Israel to be revealed.
It is for this reason that responsibility for the private actions of other Jews begins only after they crossed the Jordan together – the time that the nation truly worked as one unit. Then, when one organ malfunctions, it affects the whole body. Inversely, when things are working properly, the innate levels sparkle brilliantly, and the Torah can be followed in a complete manner. This is the idea of the binding together that is created by the unity on the earth.