Allusion and Illusion are not perfect homophones, because they don’t sound exactly alike. But they come pretty darn close. By changing just one letter, you have a different word and a different meaning. Spell-check would not catch this type of error.
Allusion is a noun. It is a literary device where the current situation references a previous one, usually in a more well-known story, myth or fable. In Judeo-Christian countries like those in Europe and the United States, biblical allusions are frequently used. In the Middle East and Africa, it would make sense for Koran references to predominate.
Illusion is a noun. It is something seen, or believed to exist, but that doesn’t really exist. For example, the lost wanderer in the desert seeing a refreshing pool of water ahead where none exists, or a forlorn hero or heroine who mistakes another person’s kindness for attraction. Both the wanderer and the forlorn hero have illusions about their situation.
The following sentence uses both words correctly:
Film fans frequently seek allusions to classic films in newer films, but sometimes this is just an illusion, a mere coincidence. After all, how can you allude to film you’ve never seen?