Religion in science fiction is one of my favorite themes when it is done well. I find the ability of science fiction to examine religion in new ways fascinating. The problem is that most of the time science fiction writers are more interested in writing very shallow religious characters or mocking it rather than really examining the ideas. The odd thing about A Case of Conscience by James Blish is that it takes the religion in the story seriously and has interesting ideas but I simply did not care about it most of the time.
The biggest problem that this story had to me was that it felt as if there was a lot of padding. Most of the arguments about what to do with this world are not all that important and getting to them takes to long. Explaining why the aliens should be slave labor to mine materials for bombs is simply not the central point of this story and really does not feel all that connected.
Where I really did enjoy this story was the examination of the aliens. Twelve foot tall lizards with pouches and a society that has no war or even crime. There are a few points that are interesting here. One of the reasons that this world appears to be peaceful is that there were not the disturbances by asteroids and other disasters. This means that the ecosystem has not be shaken up as often. It also means that there is very little metal in the world. This leads to technology which is quite different but actually more advanced in some ways. Their culture is interesting, described as a almost perfectly christian culture by one and as a trick of the devil by the other. This is mostly due to a sort of innocence because of their lack of conflict. This does not mean they do not die from violence though. The children are laid as eggs in the water and go through the different stages of life living as a fish, an amphibian, animals in the forest and finally adults. This is interesting and the details are useful.
There is a lot to like in this story and a lot not to like. Many of the characters feel flat and the second half of the story really is not all that interesting, but the aliens are interesting and the idea of religion dealing with the aliens.