According to PC Gamer there is a hidden message in Doom's soundtrack, if you listen to it backwards.
This was revealed by the game's composer himself, Mick Gordon, during a presentation in GDC 2017. The announcement was made as apparently noone had noticed it before. The message itself, Jesus loves you, is perhaps more surprising than the fact that there was something hidden in the soundtrack.
Playing music in reverse is a reference to some fundamental Christians during the 70's and 80's. It was common practice at the time to find different hidden messages when playing rock music records in reverse (at least when you REALLY were looking for them).
Some bands took advantage of this by inserting something deliberately to their records. One example of this is a Finnish metal band Tarot. Allegedly they hid a line that went "I knew you were listing this backwards you pervert!". In the world of video games there was a message if you played the Brutal Legend announcement vinyl (?!) in reverse.
It seems that the people making the latest Doom game really had a good time while making it. Do you listen to music backwards?