God mode is a state which prevent you from taking damages. To enable it, you need to drown while using bash.
To do so, stay close to an enemy while being underwater, wait until you hear Ori screaming and then hold bash. If done correctly, you will die while bashing which will grant you the bash's invulnerability when you respawn. You will preserve this state until you either bash again or exit your save file.
For example, this can be used in order to swim in dirty water, giving you access to woods without Clean Water.
If you have Water Breath you won't be able to drown, making you unable to enable god mode. For unknown reason, you also can't enable god mode if you have Spear.