One of the strangest claims I've ever heard an atheist make is that the concept of the afterlife is a new one in Christianity, and that the Old Testament doesn't mention it at all. Some Jews also believe this, and Jesus personally lived alongside a Jewish denomination, the Sadducees, who did not believe in an afterlife.
This idea is not true. As shown in Job 19:26-27, the Old Testament certainly describes an afterlife. Not only is Job an Old Testament book, but it's also very likely to be the oldest book of the entire Bible, even older than Genesis.
There are many more Old Testament verses that describe an afterlife, but they don't go into huge amounts of detail. The atheists are wrong to suggest Christianity is a developing religion as it is made up by man. It is, however, a developing religion. The Old Testament's description of the afterlife is supposed to be less vivid than the New Testament, because the revelation of scripture was continuous. God didn't just drop a book next to Adam and say "there you go, a perfect record of my dealings with humanity." Instead, the Old Testament's purpose was to point us to Jesus. The Old Testament is incomplete because it is completed by the New Testament. Nevertheless, it is obvious to anyone who takes the time to read that the Old Testament does talk about the afterlife, even in its oldest book.