Typically in fantasy media, special swords tend to be able to sense someone's person, it varies in complexity from thoughts, feelings, bravery, modesty, other traits, or a combination of many things.
Seeing this is a Holy Sword, I'd imagine it senses whether the person's goals and beliefs are pure and good, and now might be the first time the Holy Sword has sensed that he isn't doing things for personal gain, whether it be monetary or reputation, or something else. It seems like it has deemed that he's now pure of heart and good enough to be recognized and given the true powers of the sword.