The belief in one G-d is perhaps the most fundamental teaching of Judaism. Thousands have died rather than negate this central tenet of Judaism. "In the beginning, G-d created the heaven and the earth" lays down the Biblical perspective right from the beginning. All that exists in this universe has only one source. While this may seem obvious to us, it is not necessarily inherently so. Could it not make more sense to argue that good and evil are a reflection of competing gods?
It would take the powerful teachings of Judaism to bring the message of monotheism to a pagan world. And apparently, Judaism could not do it on its own. The Rambam, in a fascinating (and previously censored) historical vignette codified in his legal code (see Hilchot Melachim 11:4), notes that we can never fathom the ways of the Almighty. Thus, despite the many calamities caused to the Jewish people by the adherents of Christianity and Islam, the Rambam "rules" that Christianity and Islam have paved the road for belief in a personal Messiah and the worship of G-d.
It would take thousands of years to turn the tide against paganism. This need to assert the notion of ethical monotheism can help to explain why the Biblical text is replete with admonitions against idolatry, warning the Jewish people over and over again not to follow in the pagan ways of the nations that surround them.
The natural urge for idolatry was so great that it was only through the prayers of the Rabbis, the Talmud asserts, that idolatry was by and large uprooted from the midst of the Jewish people. During the First Temple period, king after king after king led the Jewish people in their idolatrous ways; eventually the idolatry was so deeply rooted that the Temple had to be destroyed. Yet during the Second Temple, idolatry was nowhere to be found, and Torah learning flourished; tragically, so did internal strife.
With this perspective, we can better appreciate the difficulty posed by the creation of man. "And G-d said, let us make man" (1:26). The dualism implied in the verse troubled the Sages. Why would the Torah leave an opening to say that G-d did not create the world alone? Why allow people to claim that there is more than one Deity? Yes, the belief in one G-d is most important, yet teaching values is even more important--even at the risk of causing some to deny the unity of G-d. As Rashi notes, G-d--in consulting with others (the angels) before He created man--is teaching a powerful lesson in "derech eretz and humility, that the greater one should consult with and take advice from the lesser ones". At the moment of the creation of man, G-d teaches us that our character development is even more important than our religious beliefs.
A Jew must be willing to die rather than deny belief in one G-d. This is so despite the fact that such denial is made under duress, and does not at all reflect the person's true belief.
Nonetheless, teaching proper character traits outweighs the potential danger to teaching a false belief in G-d. Thus, the Meshech Chochmah (Shemot 14:24) notes that the verse we read on Yom Kippur that "G-d dwells in the midst of our impurities" (Vayikra 16:16) is a reference to the sins of sexual immorality and idolatry. G-d's presence remains amongst us despite these grievous sins. However, the Meshech Chochmah notes that for the sins of character, gossip, and divisiveness, we drive away G-d's presence.
On Yom Kippur, we were forgiven for the sin of the golden calf. What is much harder to gain atonement for is a lack of derech eretz. If G-d is more concerned about our middot than our beliefs, so should we. As we begin the Torah reading anew, let us focus on developing our "derech eretz and humility" as we strive to emulate G-d.