Why did Okonkwo call his five sons together?

Okonkwo calls his sons together to warn them of their outcomes if they decide to follow the new religion as Nwoye did.

He sent for the five sons and they came and sat in his obi. The youngest of them was four years old.

“You have all seen the great abomination of your brother. Now he is no longer my son or your brother. I will only have a son who is a man, who will hold his head up among my people. If any one of you prefers to be a woman, let him follow Nwoye now while I am alive so that I can curse him. If you turn against me when I am dead I will visit you and break your neck.”