The screen was the loading screen for any given level, it was to show that the game would have brutal difficulty and it would be insane.
In later games Edit
The message re-appears in 2009 game Wolfenstein, as an occasional break from the general tips that appear on loading screens throughout the game allowing the fans for the Nostalgia for the old game. It also appears in the the 2014 game Wolfenstein: The New Order on the first loading screen before the intro as a subliminal-esque message. In the 2017 game Wolfenstein 2: The New Colossus the message flashes on screen right before the ending credits and it's written in bloody letters.