The desire of Christians to spread the Gospel is quite understandable, but one has to pick the proper venues.
It's seems rather obvious the Captain involved was deeply touched and otherwise affected by his missionary trip the previous week (a proverbial "religious experience"), and I'm sure he's undoubtedly a nice fellow, but his use of the left seat as a pulpit for even a basic broaching of any religious topic (of any faith) was both ill-advised and unprofessional.
Sacking him (as some others have advocated) I think is pretty harsh, and it'd be my hope that he be given some time off (perhaps with counseling) to search for (and hopefully find) that best balance between personal belief and knowing where/when to espouse it.