Terrible Triangle, or the relationship between the two central characters in the film. Indeed, there is no rational reason for the characters to be linked.
In what feels like the ultimate cheat, there is almost no definition given to the sides or conflict of the civil war. Joffé, I assume, was going for a sense of confusion on the part of those living through the events, but those characters certainly would have had much more insight than what the film allows them. Then there is the possibility that Josemaría Escrivá's (Charlie Cox) siblings may have died because of the flu pandemic of 1918, but no reason is given for their deaths. Like the rest of the film, it feels both ill-informed and halfhearted.
Essentially, this should be the story of how a man spent his adult life trying to come to terms with the actions he took during the Spanish Civil War, which he relates to his estranged son. For some reason, this story is mixed with a priest-on-the-run tale, and it is nowhere near as interesting as the real story must be. Giving no character enough room to develop, There be Dragons moves ever onward towards the reveal that has nothing to do with the priest or Opus Dei.
I found myself actively upset that this film was not done with a Spanish cast; I cannot explain why, except to say that it would have felt less artificial if the actors were at least Spanish or even speaking Spanish. The budget seems to have been spent on a few set-pieces and not on making the scant war scenes look authentic (though I was very excited to see a Lancia IZM in action, that was ruined for me with the independent movement on the turret-mounted machine-guns). The English speaking cast gives no urgency to the material, and they seem caught somewhere between a dull period piece and an ironic anti-war movie.
This is a prime example of where bad reviews are a great indication that a film should be avoided. It is largely a waste of time. Read a book about Opus Dei or one written by its founder, like The Way. Read about the Spanish Civil War. But don't expect any insight into life, the Church, Opus Dei, or the Spanish Civil War from There be Dragons. All you will get is some half-formed notions that don't develop into anything coherent.