Musar על מנחות 219:5

The sages also consider the words: על פני as alluding to the fact that חלק ה' עמו, יעקב חלק נחלתו. Other deities are part of אלוקים, peripherally connected to G–d, whereas the Jewish people who are described as "cleaving to G–d" are as closely attached to G–d as one's face, i.e. פנים. "Face to Face did the Lord speak with you" פנים אל פנים דבר ה' עמכם, (Deut.5.4). There is another reverse aspect to this "face to face" relationship, a relationship that could be called a "back to back" relationship, אחוריים לאחוריים. When G–d "expanded" into this physical world, He allowed for both kinds of attachment to man, i.e. "face to face," and "back to back." The Hebrew word אחרים symbolises this relationship, hence the term אלוהים אחרים describes the way the pagans relate to their god, which is itself an outgrowth of the סיטרא דשמאלא, the "left side" of the diagram depicting the emanations. The philosophers of the Gentiles as well as their rulers are all well aware of the existence of G–d as the Ineffable Name, and when they refer to Him they describe him as אלוקי האלוקים, "the G–d of G–ds." Their attachment to G–d does not extend beyond the level of אלוקים אחרים, i.e. what we described as אחוריים. The commandment לא יהיה לך warns us not to slide backwards into a peripheral relationship with G–d when we have been chosen to have a much more intimate relationship --a personal one-- with Him. Perhaps we can define the meaning of על פני as G–d saying that someone who has been granted access to what to others is the most hidden aspect of G–d, the אין סוף, must not squander such a close relationship by addressing himself to subordinate instruments of G–d. [Here I have paraphrased the author's remarks. Ed.]