Zen has its roots in Buddhism, which is, in essence a method for pursuing spiritual enlightenment.
The Buddhist tradition is founded upon the alleged teachings of a theoretical original teacher, Prince Guatama Siddharta, the Buddha, who is said to have achieved enlightenment in Northern India six hundred years before the common era.
According to the Buddhist tradition, Siddharta entered into a state of infinite bliss upon his 'awakening'.
His utter and complete contentment, however, was quickly tempered by the realisation that he was the only one, a situation that his boundless compassion found so unacceptable that he immediately resolved to teach others, that they may follow him.
It was at this point that he realised that there could be no Creator.
Since Siddharta was only too aware of his own moral compulsion to dedicate himself to the enlightenment of others, it was impossible, in his mind, that an infinitely powerful moral Creator could have made beings, without also feeling morally compelled to make all of those beings awakened, from the outset.
In short, our own spiritual predicament, and our need for awakening, proves that there can be no God.