Which political ideology was purely atheistic in its belief system and therefore diametrically opposed to what the majority of americans held sacred?

Communism

Explanation: The political ideology clearly atheist in its belief systems, diametrically opposed to what most Americans considered sacred, is Communism , a political movement that considered itself independent of any type of religious belief, since it did not ingest its themes for be considered neutral. and rather focused on maintaining the condition of equality for all citizens, with the intention of avoiding any type of discrimination derived from a religious issue.

This position or ideology is very close to humanism, which is based on the claim that some aspects of human nature could be solved without the help or existence of some divinity or God.

this is a good question! i'll do some research and come back when i have the answer!