Monotheism

Monotheism is elegant and practical. The core principle in Judaism is monotheism, the belief in one G*d, an undivided G*d, just One, because G*d is "the one and only". This is reflected in the Shema prayer:

"Shema Yisrael, Adonai Eloheinu, Adonai Echad. Baruch shem kevod malkuto le-olam va'ed. Listen, Israel: the L-rd is our God, the L-rd is One. Blessed be the name of His glorious kingdom for ever and all time.

Love the lord your God with all your heart, with all your soul, and with all your might. These words that I command you today shall be on your heart. Teach them repeatedly to your children, speaking of them when you sit at home and when you travel on the way, when you lie down and when you rise. Bind them as a sign on your hand, and they shall be an emblem between your eyes. Write them on the doorposts of your house and gates." (Deuteronomy 6) (translation from The Koren Siddur)

From the practical point of view, anyone can know and remember that G*d is one. For some of us, this may sound as fundamental as it can get, certainly there's only one G*d. In polytheism however, problems arise as people are not actually able to remember exactly how many gods are and what each one does. And yes, this is still a common phenomenon today.

In Judaism, G*d cannot be represented visually by images, sculptures, dolls, and related. If you want to know G*d, you do this by developing your intellect, there is learning and prayer, so you pray to G*d, you study the Torah, you do mitzvot, and so you establish a personal connection to G*d. By doing so, G*d gets to be with you, in your heart and in your mind, not in a remote place.

When people start representing G*d in visual terms, they end up sitting in front of those images or sculptures, and so they limit themselves, and they limit G*d. G*d cannot take human form or animal form or any other form, G*d is not visible and cannot be represented in visual terms because G*d is so much more beyond that.

To understand G*d, one must remember that G*d is above these labels that us, humans, create.

In order to differentiate between "monotheistic" and "polytheistic" thinking, I would compare monotheism to a type of thinking where someone learns to make good choices that represent practical solutions because they actually yield the desired results in an efficient way, whereas polytheism can be regarded as an inability to perceive things for what they actually are, making things even more complicated, which would result in a series of negative consequences, one after the other.