Correct, there is no way to train Clerics, nor is there any way to promote them. If I remember correctly, when I asked about this on the Discord chat, Frogboy said the developers couldn't think of any good promotions. Although a Recall promotion, like the ones available to other citizens, would be useful. And I think a few other possible promotions have been suggested by now. So I don't expect it to stay this way forever.
