Cloud has come to realise that people do not die.
Bodies can be corrupted, beaten in, mutilated so badly that even the soul should be worn away. They can rot slowly from the inside, and the heart can suddenly stop, sending painful jolts through your every nerve as if to say its final 'goodbye'.
But people do not die.
Cloud can vividly remember the look on Sephiroth's face as his sword took its final slash at the demi-god's tainted skin, and the way the unholy light drained from his eyes. Cloud knows that at some point - tomorrow as soon as he wakes, next week, in twenty years... - that man will be back, and he knows he'll have to bathe him in his own crimson once again.
But this is not what Cloud means.
Every morning when he wakes he finds himself in that church - her church - and knows more than ever that people do not die. If his mind struggles it can rewind, and for a fleeting moment she's there again, peacefully amongst the flowers. Sometimes she even turns to him, but never once has he been able to fix his eyes on her, and so she fades unlike the memory refuses to. When he traces invisible lines in the sacred waters he can feel her will radiate from each and every ripple, and when he really thinks about it he can feel something tugging at his still-beating heart. The painfully beautiful memory doesn't fade in time; it never wears away like the torn old photos of her flower field- it's more like the flower field itself. Every time winter comes and freezes over, and it seems death himself has visited, the spring is promised, and all the colours are in bloom once more. This is what Cloud means. People never really die.
Aerith Gainsborough never died.