Zen priests often write Zen phrases on hanging scrolls or shikishi, square boards traditionally used for calligraphy. In Japan, the calligraphy of Zen priests is often referred to as bokuseki (墨蹟). I have always felt that these works are deeply connected to Zen.
According to Togami Myodo, a Rinzai Zen priest, the calligraphy of Zen priests differs from that of professional calligraphers. In Zen, the question is not simply whether a piece of writing is technically better or worse. The brushwork is also a means of personal expression, reflecting the spirit of Zen.
Even so, I think many Zen priests are highly skilled calligraphers. I admire both the character of their writing and the feeling it conveys.
