Monotheism came from Zoroastrianism
The Babylonian captivity was a turning point in religious thought exposing the Hebrew intellectual elites to concepts which were developed in more Eastern areas, notably Zoroastrianism. Though Zoroastrianism gained the status of "state religion" in Persia only later on, the basic concepts were older (the exact date is highly disputed, but the current consensus points to "some time in the 2nd millenium BC").
The ideas had begun to percolate to neighboring Babylonia at the time the Hebrews were there. Among these concepts was the notion there was a Supreme Deity (Ahura Mazda). All other "gods" were really subordinates, even proxies; every prayer sent to any god was ultimately brought to the attention of Ahura Mazda.
When the Hebrew returned from Babylon, monotheism crystallized in their minds: they now understood that a god could be fake, non-existent. A god doesn't need a statue or a totem; if you can think about it, then it exists, and thus you can worship it.
Since monotheism was a gradual innovation, there are "intermediate states" and one cannot really pinpoint an exact year in which it happened. The term henotheism has been coined to describe these intermediaries. In the case of Europe and Middle-East, it seems that true monotheism emerged with Judaism in the 6th or 5th century BC, although some definitions of monotheism can include earlier Zoroastrianism or Atenism (as described by @Semaphore).
Well let’s go in and see what’s on display.” Katie walked into the first chamber, a square room with a statue of the ancient pharaoh displayed in a large glass box in the center of the room. It was a commanding presence. Impressed she slowly circled the statue. “I’m surprised how soft his face is.” It was a life-size bust of a man with an elongated face, full, almost feminine lips, the eyes with an oriental slant.
“Where did his oriental features come from, Neal?”
“Akhenaten’s mother Tiye was the daughter of Yuya and Tjuyu, both Syrians living in Egypt. They were probably Mitanni because they were into horses, which were still foreign to the Egyptians. Tiye’s dad, Tjuyu was put in charge of horses and cattle for the Egyptian government”
“I thought the Egyptians were already into horses, wasn’t the Battle of Kadesh a big chariot war?”
“Yeah, but this was a hundred years before. It was during this time the Egyptians came to realize the awesome power and speed horses brought to the battlefield. That’s probably why a Syrian couple living in Egypt became so important. They even fathered Ay, who became a general and a pharaoh himself.
“Akhenaten influenced art as well as religion,” Neal continued. “His artwork shows a softer side of life than previous imperial statues and reliefs have shown. He was definitely making a new statement during this time.”
Tal came in from an adjoining room. “This next room is full of tablets, almost like a library.”
“Here is the Amarna Room,” Neal said. “These are the clay tablets Akhenaten used to correspond with the other kings and government officials throughout the ancient world.”
“How big was his world? How many people could he have written to?”
“Well Katie, the clay tablets, there are more than three hundred of them, indicate some of his correspondents was from empire called the Hittites, in what is now modern day Turkey. They became increasingly assertive during Akhenaten’s rule, going to war against the Mitanni beneath them in Syria and Iraq, who were an Egyptian ally. In addition to their conflicts with the Mitanni, the Hittites were also stirring up instability in the vassal states of Syria, and helping a Semitic nomadic group, the Apiru, to create unrest in Syria-Palestine.”
“All of this was going on three thousand years ago?”
“Yes, about then. And the Old Testament deals with a lot of this stuff too. The information we get from the tablets is corroborated in the Bible.”
Katie was fascinated. This stuff intrigued her to no end, but when she looked at Tal, she could tell he wasn’t at all enthralled.
“What else did you find in the other rooms, Tal?”
“Just more stone stuff. I was hoping for mummies and chariots and lots of gold.”
Amenhotep III and Queen Tiye
Akhenaten and his family lived in the great religious center of Thebes, city of the God Amun. There were thousands of priests who served the Gods. Religion was the business of the time, many earning their living connected to the worship of the gods.
All indications are that as a child Akhenaten was a family outcast. Scientists are studying the fact that Akhenaten suffered from a disease called Marfan Syndrome, a genetic defect that damages the body's connective tissue. Symptoms include, short torso, long head, neck, arms, hand and feet, pronounced collarbones, pot belly, heavy thighs, and poor muscle tone. Those who inherit it are often unusually tall and are likely to have weakened aortas that can rupture. They can die at an early age. If Akhnaton had the disease each of his daughters had a 50-50 change of inheriting it. That is why his daughters are shown with similar symptoms.
Akhenaten was the son of Amenhotep III and Queen Tiyee, a descendent of a Hebrew tribe. The largest statue in the Cairo Museum shows Amenhotep III and his family. He and Queen Tiye (pronounced 'Tee') had four daughters and two sons. Akhenaten's brother, Tutmoses was later named high priest of Memphis. The other son, Amenhotep IV (Later to take the name Akhenaten) seemed to be ignored by the rest of the family. He never appeared in any portraits and was never taken to public events. He received no honors. It was as if the God Amun had excluded him. He was rejected by the world for some unknown reason. He was never shown with his family nor mentioned on monuments. Yet his mother favored him.
A replica of the Tuthmoses IV Sphinx Stele.
The Sphinx was seen by ancient Egyptians as a representation of an ancient aspect of the sun god, Horemakhet, which means Horus in the Horizon. On this stele Tuthmose IV makes it clear that he owes his throne to the intercession of Horemakhet and no mention is made of Amun. In part the stele reads;
“Look at me, observe me, my son Thutmose. I am your father Horemakhet-Khepri-Ra-Atum. I shall give to you the kingship [upon the land before the living]….[Behold, my condition is like one in illness], all [my limbs being ruined]. The sand of the desert, upon which I used to be, (now) confronts me; and it is in order to cause that you do what is in my heart that I have waited.” (Stele of Tuthmose IV)