The Khmer Rouge, or the Communist Party of Kampuchea, used this flag:
It features the Angkor Wat, a famous Buddhist temple with a Hindu origin.
As far as I have been able to understand, the symbol was chosen as it was seen as a symbol of the nation. I find even this slighly unsual for a communist movement, as such entities often seemed to idealise international worker's fraternity rather than nationalism.
Communism traditionally viewed religion as something negative and dangerous. It seems that this is the only communist regime/party/nation to use a religious image in their flag? While many other communist entities have resided in areas with a history of religious symbolism in their flags, coat of arms, etc, most of the communist entities did not keep them.
For example, if the Russian SSR had used a church on their flag, I would find that strange.
What made the Khmer Rouge different? Are there any sources documenting a discussion, controversy or disagreement on this? Can we get any insight into their reasoning for using the temple in the flag? If the reasoning was something like the icon being a previous symbol of the nation, are there any sources showing why this was thought to be more important than communist ideals? Is there any research or sources showing how this case differed from other communist entities that did shun religious imagery?