Cold, impotent ash.

"Living fire begets cold, impotent ash."

Out of everything in Things Fall Apart, this made me think the most. It's something I have always pondered on, how sometimes parents and children are opposites. The way Achebe portrays Okonkwo's view of his son is slightly disconcerting, but it suits his character.
Okonkwo believes he is the greatest of great, having slayed five men, and having overcome his birth. When a stranger's son resembles him more then his own son, Okonkwo tries to change his son. He isn't an affectionate person, and he just comes off as brute-ish.
When his son leaves with the church, Okonkwo struggles to deal with it, although he attempts sort of an 'oh well' attitude.

I believe the quote represents the way that Okonkwo rationalizes his loss. He believes it was pre-destined, that there was no other way that his son could have turned out.