How was idolatry possible for the Jews prior to the Babylonian captivity, when that’s the point at which they inherited monotheism from the Persians (from Zoroastrianism)? That’s also the point at which the Zadokite priests returned to Jerusalem and compiled and redacted the Jewish scriptures from a strict monotheistic perspective, with the benefit of hindsight, labeling as idolatrous the Canaanite polytheism that had been native and standard practice at the time for Jews.
Do you really think people suffer in life only because they deserve it, because they go against God? Ecclesiastes and the Book of Job answered that naïve form of zealotry for the Jews. If you’re going to delve into Judaism to support Christianity, you’d have to take those wiser parts of Judaism into account. The upshot is that lots of suffering is unfair, as far as any informed person can tell in good conscience.
I have an alternative account of the greatest sin of Christianity. It begins by noting the exquisite irony of your suggestion that this sin is idolatry and of your using Judaism to illustrate this, since Jewish monotheists regard the Christian worship of Jesus as perfectly idolatrous. Here, too, there was an “absorbing of pagan practice,” with syncretism between Judaism and Greco-Roman polytheism (with Mystery religions, the dying and rising savior gods, Stoicism, Cynicism, etc). And the one transcendent God became three divine persons, including a human man.
This accounts for what you and the New Testament call “lukewarm” Christian faith. The reason so many Christians are lukewarm is that it takes appalling audacity or what the Jews call “chutzpah” to maintain strong Christian faith in light of these ironies and absurdities at the base of the Church's cooptation of Judaism and its formative alliance with the Roman Empire (with the empire that slaughtered its spiritual founder). Christians are lukewarm because they dimly perceive, at least, the brazenness of the Christian creed and they lack the stomach to take it seriously.
The greatest Christian sin, then, would be the retreat from that recognition of Christianity’s absurdity, into zealotry that harmonizes the conflicting passages of the Bible, makes theocratic excuses for the brutality and ungodliness of one “Christian” empire after another, rationalizes Evangelical support for “President” Trump, and so on and so forth. It’s not the lukewarm Christian that adopts secular idols, but the fundamentalist who’s crazy enough to believe anything. Lukewarm Christians are functional atheists and secular humanists.
The main Christian sin, then, isn’t idolatry but the “strong faith” that sustains the pretense that Christianity is something other than the idolatrous, preposterous mashing together of monotheism and polytheism.
