Colin, I think a lot of Western haiku poets like to use suggestion and are not open to speaking or writing in a dogmatic air, so perhaps the Christians among us in our "haiku world" are not as apparent, but they are there. And of course there is an overlap of people from many faiths who are open to discussing religion and what is their worldview and so on. Certainly, almost all Western haiku poets have been influenced by Japanese culture in one way or another.
In the political front (at least in the US) discussion of religion has become as fearsome as the times of Henry VIII. Perhaps, most poets prefer gentler discussions. I certainly wish it was as easy to discuss religion as it was in my college days in Michigan. Later, when I studied, worked and lived in Japan (31 years), I found very few Japanese wanted to discuss religion deeply. It was a surprise for me at first. I thought, deciding whether you believed in God or not was a part of becoming an adult. Most of my Japanese friends followed their cultural practices like praying to/for their relatives who passed on, giving money to temples at New Year's and taking part in Shinto Festivals or Obon Celebrations. However, none of my friends went to temples frequently, the way my Japanese Christian friends attended church or mass. Not the way, that is common in some Southeast Asian countries like Thailand. There the temples are a big part of the communities, young people hang out there, and the young Thai men usually study and work at a temple in their youth.