Let me start by giving the full context of the quote: “From now on, it is our task to suspect [everyone] among us. Forewarned is forearmed. Take no risks and be alert to danger.”

Though this is quoted in Agatha Christie’s novel, the saying is originally Latin (“Praemonitus, praemunitus”). The meaning of the phrase hasn’t changed much over the years—it advises that you stay aware of your surroundings, because being aware of a possible danger allows you to prepare for it. After all, it's difficult to defend yourself against something you don’t know about!

In the context of the novel, this is certainly good advice. When this phrase is uttered, the characters have arrived on Indian Island and are being killed off one at a time. At this point in the story, the remaining individuals in the group are being warned not to trust each other, as no one is sure who is responsible for the deaths. The lack of information means that there is no concrete course of action that the group can take to protect themselves—the threat is largely unknown and will only be revealed in the heat of the moment. The only realistic protective measure available is for every member of the group to be suspicious of those around them. If you expect the unexpected, you can never be taken by surprise.