He was a Zen Buddhist (in the Soto Zen tradition, his teacher was Kobun Chino Otogawa). I wouldn't outright say he was an "atheist", although I'm sure some might disagree
It is a thankfulness meditation, which is a basic ingredient for feeling content. And rich people need to feel content as well. It is sort of like prayer, but to actual real beings. That's why I thought it could be an atheist manifesto.
> The rich are no different to the poor. They can't take their worldly possessions with them to the next life.
This holds true, assuming that there's an afterlife of any sort.
If there isn't, then the rich would simply live lavish lives, before ceasing to exist. Same outcome as for the rest of people, rather different experience during their lives.