The following image I assembled breaks down the debate between Christians who accept old-earth theories such as the Big Bang and evolution and those who do not to the very core of the issue:
Most Christians who accept old-earth will also maintain that the Bible is a result of human error, full of contradiction, and relies on faulty translations. This is fodder for atheists! They have just undermined their entire belief system. Through a series of simple questions, opponents can break the faith of old-earth Christians to ridicule in minutes. Although they obviously disagree with our conclusions, atheists already know that YEC is the only Christian worldview that is internally consistent and defendable.
If the Bible is the word of God, then it only makes sense that we would use that to compare all other truth claims to. If it is the word of man about a god, then it is not worthy of second thought.