Rambam describes a three-step process leading to the inception of idol worship, which he asserts was the result of a protracted degeneration of faith in God’s unity.
Initially, people saw how the divine energy flowed into the world through various intermediaries.
This eventually led to their taking note of those intermediaries, as they felt it necessary to forge relationships with them.
(This is similar to earthly kingdoms, where people make it their business to establish a good rapport not only with the highest-ranking official, but also with his subordinates.)
As time passed, the intermediary gained increasing prominence, while the primary source progressively lost His.
The distance between people and the source increased, and their understanding of God weakened, until eventually the primary source became a relic of the past.
People not only ceased relating to God, but their original belief in a transcendent cause of existence was utterly lost.
It is thus a reality that purportedly monotheistic religions may become idolatrous, albeit in the guise of venerating local saints.
Should this occur, man loses all vestiges of any previous connection with God.
Rabbi Adin Steinsaltz