Is Religion the opium for the mes, or the tool of revolutionaries?

I believe that most religions, at their core, advocate good. However, there is a huge difference between religion and spirituality. We all have spiritual needs at some level - that need to understand and feel connected to the things which are greater than and/or beyond us. Religions are constructed by humankind as a way to tap into and meet our spiritual drives. Because religions are human constructed, they are also easy tools for human manipulation. That, I believe, explains the examples you give. Some choose to use religion in a positive way; some choose to use it for their own gain, as a tool for subjugation or an excuse for terror. I think your vehicle ogy is apt. As for Marx, we are all victims of our own observations and we are all subject to confirmatory bias - we tend to see the things to support what we already believe. Marx was no different.