The Light will not abandon you, you may only abandon it.
There is no moment where the light will not come if it is asked for, there is no point low enough to be out of reach, but you have to let it find you.
A fallen Paladin of the Light does not exist, as there is no location beyond the reach of the Light, yet it is only as strong as the paladin makes it; while the light may reach even the darkest of corners in the deepest of dungeons, it cannot reach a paladin if they are hiding from it, and can do nothing to empower them if they are to forsake it.
The strength of the Light lies in those who have faith in it never leaving them, as they understand the choice to do so is theirs alone, and it is they who decide if it can reach them where they dare to tread.
An unworthy paladin of the Light is a paladin that has decided to be so, not one that the Light has left, as the Light will find them even in the darkest of hours.