i think you're getting a bit confused here. the idea that judaism meant monotheism goes back to abraham at the minimum. abraham is in fact considered by us to be the first jew and, in many ways, the first monotheist. abraham pioneered the monotheist's relationship to G!D in contradistinction to the power/reward relationship favoured by his polytheist contemporaries. the trouble is that it took such a long time to get the message out that we really really mean it about the monotheism - and the no image rule. abraham's family intermarried with many polytheists (i suppose it was kind of unavoidable) and indeed polytheistic was a feature of our formative period as a people in egypt - it took us a very long time indeed to rid ourselves of those bad habits, as the golden calf episode ought to show. similarly, most of the people indulged in non-monotheistic beliefs and practices throughout the period of the judges and the early kings. what i think you have not understood, however, is that monotheism is by definition what judaism was always trying to get across; it was never acceptable in judaism to be polytheistic, but it was very much nonetheless the reality of people's lives, to the despair of the prophets, whose message is beleaguered and aggrieved but defiant in this context. monotheism was the aspiration and the goal, but rarely, at least until after the destruction of the first Temple, the everyday normality.
polytheism was never something that was acceptable according to the Torah and it is this that we must remember, not seek to hold up the actual behaviour of the people as representative of what was right, as it says: "in those days there was no king in israel; everyone did what was right in their own eyes".
b'shalom
bananabrain