Although Zen is a spiritual tradition, there is nothing spiritual about Zen. There is no specific god in Zen. Buddha was a man, who found a way to live with his mind in such a way that he didn’t suffer. He then set about teaching others how to live with their minds. Living with your mind may include worshiping a god or making offerings to the dead. It may include passing from one life to the next. It may not.
The spirit of Zen is the spirit of life. It is nothing more than all of existence and nothing less. If you bow to a statue of the Buddha in your practice, it is not because Buddha is a god and you are something less, it is because you are bowing. If you sit in meditation, you are not any closer to the spirit of the universe than when you sit on the toilet. Both of those activities are part of life.
In god fearing cultures, it is hard not to imagine a conscious power pulling strings to make things happen. Even atheists may find themselves asking the cosmos for favors in a pinch. Whether God exists or not is beyond the scope of Zen. Not knowing is the spirit of Zen. What is God? What is Buddha? What am I? Don’t know. That’s the spirit.