The quote that this question refers to comes at the end of Chapter 19, which is also the end of the second part of this compelling novel. The elder has just been surprised at the largesse and generosity of Okonkwo's feast that he has held for everybody, and he makes his thanks public in the speech, holding up Okonkwo's munificence of an example of respecting the old values and traditions of the tribe. He says this:
But I fear for you young people because you do not understand how strong is the bond of kinship. You do not know what it is to speak with one voice. And what is the result? An abominable religion has settled among you.
The elder is therefore refering to the way in which the tribe is becoming divided and is splitting over the presence of Christianity among them that seems to do so much to shake and move the tribe away from its traditional cultural roots and values. If you look at the speech in detail, you will see that the elder gives specific examples of how Christianity is viewed, in his eyes, as a threat to the unity of the tribe. The fact that the tribe does not now "speak with one voice" is therefore seen as a profoundly negative thing, as it weakens the tribe and brings the extinction of their values and beliefs one step closer.