Press '~' to access the console then enter 'iamtheone' to enable Cheat mode. Enter the following codes to activate the corresponding cheat
God Mode: god
No Clipping: ghost

Enable Flying: fly

Disable Fly: walk

Suicide: Suicide

1st Person View: behindview 0

3rd Person View: behindview 1

Slow Motion: slomo
Disable Slow Motion: slomo 1
Invisibility: invisible 1

Disable Invisibility: invisible 0