The way I read this, Radiant Soul wouldn't apply to this since the spell guardian of faith is actually about summoning the guardian, not dealing the radiant damage directly.
From PHB, pg. 246-7:
A Large spectral guardian hovers in an unoccupied space of your choice that you can see. It occupies the space and is indistinct except for a sword and shield that have the symbol of your deity.
The guardian is the one then dealing the damage, as pointed out in the emboldened part of your quote. Hence it would still only deal it'sits 60 points of damage before vanishing.