In Maimonides view, idolatry is the result of an ongoing degeneration of faith in God's oneness.
At first, people perceived God's power as it streams in this world through intermediaries.
The people focused on the intermediaries themselves.
They believed it necessary to build a relationship with them (just as it is politic to develop a relationship not only with the king but also with his subordinates).
As time passed, the intermediary began to seem increasingly important, while the primary source-God-seemed decreasingly so, as the distance to that divine source grew and the ability to reach it waned.
In the end, the primary source lost all meaning as people ceased to relate to it and even stopped believing that somewhere in the heights, a primary Cause of all being exists.
Thus even religions nominally described as monotheistic may engage in the adoration of their local holy figures to such a degree that only their name distinguishes them from idolatry.
At that point, the connection between man and God has been severed.
–Rabbi Adin Steinsaltz
From Opening the Tanya by Rabbi Adin Steinsaltz