Welcome
People don't become atheists. They ARE atheists (or I would say agnostics), and they become Christian. Everyone is born in rebellion against God, it is a change of heart that makes anyone different. So, that is very helpful for me to remember when talking to them, because it keeps me humble and keeps my mind and heart focused on God.
Usually I feel pretty sick when I hear someone talking like that. It is horrifying, and most of the time they don't even hold that opinion themselves, they're just spitting back information they've heard.
I've always viewed silence as agreement. So personally, if it is the place and time to do it (not gonna interrupt someone or be obnoxious about it), I will always say something if something bad is said to me about the Bible. It doesn't have to be much. Remember that you're defending the Bible, not yourself. Sometimes a simple "I read the Bible, and I think it's true; do you want to know why?" Is cool. Be prepared for ridicule and rejection, because people don't want to hear it.
Look up different facts that prove the Bible's history. You'll never make someone else believe something unless you're firm in your own beliefs about it. READ YOUR BIBLE! So many young Christians forget this step. If you don't know the material, how do you expect to refute errors, and if you haven't been getting your spiritual nourishment from it, how do you expect to have the strength to do so?
Lastly, know that it is God who changes hearts, and not you. You'll never change someone's heart. You won't even change their mind, most of the time! So as soon as possible, get on your knees and pray for that person and the seeds that you planted. God has promised that his word will never go out and come back empty.