The worst moment of Nagamoto's life happened on the day of Rin's awakening.
He had been terrified when his friend and mentor Shiro had suddenly burst into flames.
Petrified when a Gehenna gate was opened and threatened to drag Shiro and Rin into hell.
And absolutely grief stricken when the paladin took back partial control and committed suicide in an attempt to save his adopted son.
No word, though, could describe what Nagamoto felt next as he witnessed Rin draw his sword. His world had exploded in electric blue and for a moment the young priest thought Satan had returned with a vengeance. Then the flames cleared slightly and Nagamoto's eyes widened as they took in the sight of a demonic Rin. His heart stopped for a second thinking that the teen had reverted into a mindless beast; the same boy he had raised along slide everyone else and had grown attached to. That notion disappeared the next instant however as Rin slashed at the gate, destroying it. He sheathed his sword dousing the blue fire that had wreathed him.
Time stood still as all the priests, Nagamoto included, lay on the floor, eyes transfixed by Rin. The young demon who now bore fangs, pointed ears, and a tail knelt over Shiro with his eyes screwed shut, tears streaming down his face.
If it was possible, Nagamoto's heart dropped even more as he watched Rin break down in anguish and sob uncontrollably, his poor frame shaking. Their church was a huge, shambled mess. Shiro was dead. Rin was awakened and now in danger. He could no longer live with them. And to make matters worse Yukio picked that exact moment to walk in and start demanding answers.
People commonly whine about having 'the worst day of their life' when things (usually not even all that bad) don't go their way. He wasn't one to exaggerate, but Nagamoto was pretty certain that no other day could go as horribly wrong as that one and the man was convinced that he had lived through the worst day of his life.