AThe common belief is that the apple symbolizes the forbidden fruit, spoken of in the Bible. The apple also is a symbol of the death note. When Light used the death note, it was like he partook of the forbidden fruit. There are multiple references such as this one, to Christianity in Death Note.In an interview with the artist, he used an apple because it was a common object, but also very stark stylistically. He was asked directly about religious symbolism and denied it, as if the idea didn't occur to him. The interview can be found in one of the mangas but I don't remember which one.