Essentially, the danger of forgetting the past is the first step to reliving it. A quote which speaks to this idea is as follows.
Elie Wiesel (Night)- “To forget the dead would be akin to killing them a second time.”
What Wiesel means by this is that we, as a global and/or societal whole, cannot forget our histories. In some senses, the forgetting of a history will insure that everything which has come before us came in vain. If one forgets his or her history, the chances of reliving it compound. Think about it this way: if we do not learn from things such as the Holocaust, we will re-live it (illustrated by Syria and the use of chemical weapons).
As for this idea tying to Chinua Acheba's Things Fall Apart, the introduction of Christianity into the village of Umuofia brings about the possibility of some villagers forgetting their heritage. As the novel illustrates, some of the villagers converted, forgot their ancestral history, and conflict arose in the village. When one forgets his or her past, he or she will fail to remember the things which tie a culture together. When this happens, a culture can become lost. Therefore, if one forgets his or her past, he or she can forget where they came from and feel alienated by his or her own society.