God mode

From KeenWiki

God mode is a common feature in Apogee/id games, and allows the user to participate in the game world without any risk of being killed. It is often very useful for testing levels, as it can save a large amount of time by allowing the player to walk past enemies without spending time shooting them or otherwise trying to get past them unharmed.

See the cheat section under the relevant episode for details on how to enable God mode.