Warlocks with the Celestial patron (Xanathar's Guide to Everything, p. 54-55) gain the Radiant Soul ability at 6th level:
You have resistance to radiant damage, and when you cast a spell that deals radiant or fire damage, you can add your Charisma modifier to one radiant or fire damage roll of that spell against one of its targets.
The guardian of faith spell is also added to the warlock spell list (but not automatically known) for warlocks with the Celestial patron. The spell description says:
A Large spectral guardian appears and hovers for the duration in an unoccupied space of your choice that you can see within range. The guardian occupies that space and is indistinct except for a gleaming sword and shield emblazoned with the symbol of your deity.
Any creature hostile to you that moves to a space within 10 feet of the guardian for the first time on a turn must succeed on a Dexterity saving throw. The creature takes 20 radiant damage on a failed save, or half as much damage on a successful one. The guardian vanishes when it has dealt a total of 60 damage.
If my Celestial warlock has a Charisma score of 20 (and a corresponding modifier of +5), can his guardian of faith deal 65 damage in total rather than 60?
Best Answer
No, nor would it increase damage done
It's debatable whether or not the spell guardian of faith is a "spell that deals radiant or fire damage." (NathanS covered this well in their answer.) But one thing is certain: it does not involve a "radiant or fire damage roll."
From PHB, pg. 247 (bold added):
Neither the damage dealt, nor the damage limit are determined by a "damage roll." They are set values. So this feature has no effect on either of them.