In the Imitation Game (2014), Alan Turing finds out that his associate John Caincross is a Soviet spy after reading a passage bookmarked in the Bible.
I know it was explained earlier, but what was it exactly that blew Caincross's cover?
Hugh Alexander was able to crack the encrypted message and he informed Alan about the importance of Mathew 7:7.
At the beer hut, Hugh tells Alan that he cracked the encrypted message – “Ask and it shall be given you; seek and ye shall find. Matthew 7:7.” He knows that Alan is not the spy because he would not have used a simple Bible quote for his code.
Peter harbors animosity towards Alan for letting his brother be killed despite knowing in advance. He knocks his books over and while retrieving them on the ground, Alan spots John Cairncross’ Bible. He opens it and realizes that it earmarked to Matthew 7:7. John notices Alan making this discovery, now aware that John is the Soviet spy.
See also related post.