Commentary for Kiddushin 61:2

"And the earth is My footstool": So one who transgresses in private is saying the Omnipresent is not here. Hence he is hidden, saying, "He does not know." It comes out that it is as though he is pushing away His feet and shortening Him, saying, "[He is] not here."

Anyone who transgresses in private is as though he is pushing away the feet of the Divine Presence: That is when he is capable of overcoming his inclination, and as it is shown in the first chapter of Chagigah (16a).

This one hits home hard. Definitely written by a parent well-versed in eliciting guilt.

Since the previous source imagined God dwelling among human beings, these sources continue to discuss the ramifications of God actually living on earth. The first ramification is that God is everywhere—one cannot hide from God. Sinning in secret is akin to making the theological statement that God is not in a certain place. I should note that there are sources that contradict this and seem to say that sinning in secret is preferable to sinning in public.
The second two statements deal with the proper posture and clothing to acknowledge that God is only slightly above one’s head. This last statement is one of the sources used to prove that Jews must keep their heads covered.