It goes without saying that Judaism is one of the world’s greatest religions and without her sacred Scriptures, the Old Testament, Christianity as it has been known for two-thousand years, would probably not exist; or at least in the form that it is now practiced. The point is not whether “Judaism” is monotheistic in its concepts and theology, but rather if the Children of Israel ever totally embraced it in ‘practice,’ as opposed to the religions of the other nations that surrounded them.
It seems from the following Scriptural references that it might have taken a long time for the Jews to have adopted ‘monotheism’ and belief in the God of Moses, exclusively. Eventually, the divided kingdoms of Israel and Judah were taken into captivity among the Assyrians [Israel] and Babylonians [Judah]; and even then, it is uncertain if they worshipped solely the “One and only God” before they were taken captive to foreign lands or not.
Exodus 15: 11
“Who is like Thee among the gods, O Lord? Who is like Thee, majestic in holiness, Awesome in praises, working wonders?”
“Now I know that the Lord is greater than all the gods; indeed, it was proven when they dealt proudly against the people.”
20: 1, 1a, 3
Then God spoke all these words, saying, “I am the Lord your God, ‘You shall not have any gods before Me.”
Joshua 22: 22a
“The Mighty One, God, the Lord, the Mighty One, God, the Lord!
WARNINGS AGAINST IDOLTARY, SERVING OTHER GODS, MAKING MOLTEN IMAGES:
2: 11-13 (Baals, Ashtoreth, other gods. . .)
3: 6-8 (Baals, Asheroth)
6: 25-26, 30-32 (Asherah, altar of Baal)
8: 33; 9: 4 (Baal-berith their god)
9: 37 (“diviners oak”)
9: 46 (temple of El [Baal]-berith)
10: 6, 13-14 (Baals, Ashtoreth, gods of Aram, Sidon, Moab, Ammon, and Philistines)
11: 24 (the god of Chemosh)
17: 1-6, 14, 24 (Micah’s molten images, household idols. . .)
18: 20; 19: 13(Michal, King Saul’s daughter substitutes a household idol in David’s place to protect him).
11: 1-8 (King Solomon sets up altars and high places to Ashtoreth [Sidonians], Chemosh [Moab], Milcom [Ammonites], and Molech [Ammon])
12: 25-33 (Jeroboam sets up altars for 2 golden calves; one at Bethel and the other at Dan)
14: 1, 9 (Abijam, son of Jeroboam made a molten image and other gods. . .)
14: 21-24 (Rehoboam, son of Solomon [“in Judah”] built high places, sacred pillars, Asherim, and had male cult prostitutes)
15: 8, 11-14 (Asa, son of Abijam [“in Israel”] put away the male cult prostitutes, removed the idols, deposed Maacah the Queen mother; and cut down her horrid image of an Asherah, but he did not remove the high places [“pagan worship sites”])
16: 29-33 (Ahab, son of Omri king of Israel, erected a house of Baal and made an Asherah)
1: 1, 16-17 (King Ahaziah of Israel sent messengers to inquire of Baal-zebud, the god of Ekron regarding his illness)
2: 10: 18-30, 36 (Jehu king of Israel slew the prophets of Baal, broke down the sacred pillar of Baal and the two golden calves at Bethel and Dan)
14: 1, 4 (Amaziah son of Joash king of Judah became king but he did not take away the high places; and the people still sacrificed there)
15: 1-4 (Azariah son of Amaziah king of Judah became king but he did not take away the high places; and the people still sacrificed there)
15: 32-35 (Jotham son of Uzziah king of Judah became king but he did not take away the high places; and the people still sacrificed there)
16: 1, 3-5; II Chronicles 28: 23 (Ahaz son of Jotham king of Judah became king, made his son pass through the fire, sacrificed and burned incense on the hills and under every green tree)
18: 1, 4
(Hezekiah the son of Ahaz king of Judah became king and broke down the sacred pillars, cut down the Asherah, and broke in pieces the bronze serpent that Moses made (Cp. Numbers 21: 9) because the sons of Israel burned incense to it and called it “Nehustan.”)
21: 1-7(Manasseh son of Hezekiah king of Judah became king and erected altars for Baal, worshipped the host of heaven and built altars for them, made his sons to pass through the fire, used divination, witchcraft, mediums, spirits, and set the carved image of an Asherah in the house of God) NOTE: Manasseh repented (Cp. II Chronicles 33: 11-17)
22: 1; 23: 1-15, 19-20-26 “JOSIAH’S RELIGIOUS REFORMS”
(Josiah son of Amon king of Judah’ becomes king;
and brought out all the vessels for Baal, Asherah and the host of heaven from the temple of the Lord, burned them and carried their ashes to Bethel; brought out the Asherah from the temple of the Lord to the brook of Kidron [Jerusalem] ground it to dust and threw on the graves of the common people; he did away with all the idolatrous priests; he broke down the houses of the male cult prostitutes which were in the house of the Lord where the women were weaving hangings for Asherah; he defiled the god Topeth which is in the valley of the son of Hinnon; he did away with the horses which were given to the sun and he burned the chariots with fire; he broke down the altars, smashed them and threw the ashes in the brook of Kidron; he broke down the high places and broke in pieces the sacred pillars and cut down the Asherim and filled their places with human bones; he broke down the altar that was at Bethel, demolished its stones, ground them to dust, and burned the Asherah, and he burned human bones on the altar).
17: 1, 6
(Jehoshaphat son of Asa king of Judah became king and he removed the high places and Asherim from Judah)
(Jehoiada the priest along with the people went to the house of Baal and tore it down, broke his altars in pieces, and killed Mattan the priest of Baal)
29: 1; 30: 14; 31:1 (Hezekiah son of king Ahaz king of Judah became king and caused a spiritual revival; many cast their incense bowls into the brook of Kidron [Jerusalem], and the people went to the cities of Judah and broke the pillars in pieces, cut down the Asherim, pulled down thee high places and altars throughout all Judah, Benjamin, Manasseh, and Ephraim until they had destroyed them all)
It is clear from these passages as well as the Prophet Jeremiah's denunciation of the children of Israels idoltary in making offerings to the "Queen of Heaven" that they were not wholly Monotheistic and faithful in keeping the commandments of God as delivered to them by the great Lawgiver Moses.
It was only after the people returned from a seventy year exile in Babylon to their former homeland in the land of Judah that they began the more lasting reforms [mentioned more or less in the books of Ezra and Nehemiah] of the religion known as "Judaism;" which has continued to this day.