The Previous Rebbe once related that when bombs would drop on Warsaw during WWII, all the Jews would go into hiding. Once, a whole group gathered in one room: the Rebbe, average people, simple people, and those who considered themselves to be completely disconnected from Judaism. When a bomb blew up nearby, they all yelled out in unison: "Shema Yisrael (Hear O Israel...)."

The reason for this was that it stemmed from the deepest level of their soul, the yechidah. And although their proximity to the Rebbe would only explain what awakened them to yell, the actual yelling of the Shema came from their own yechidah.

Likkutei Sichos, vol. 2, p. 334