I served as a missionary from 1995-1997 in eastern Europe and this true event occurred the winter after I came home. My older brother was a missionary in Saratov and came home in 1996. So as you can imagine, this movie was fascinating to me and I felt loosely connected to it. Take my review with a grain of salt as it's not totally objective.
The performances from the 4 main actors were exceptional. Less important roles were not as convincing and I cringed a couple of times during the scenes with parents.
The story is amazing. In order to avoid spoilers I will just say that the psychology and spirituality explaining the decisions that were made were interesting and inspiring. These 2 young missionaries embodied what I was trying (and mostly failing) to become as a missionary.
One thing that surprised me was how heart-pumpingly suspenseful some of the scenes were. The emotional intensity of the situation was conveyed very well.
The main themes were decidedly Christian - submission to God's will and the doctrine of Christ. These themes could be perceived as being delivered with a slightly heavy hand. However, kindness, compassion, and goodness in dark circumstances were universal themes and are uplifting to everyone regardless of any kind of ecclesiastical affiliation.
In short - basically the best movie I've seen all year, which is saying a lot since I saw Despicable Me 2.