How do you answer an atheist?  Learn to identify their arguments which include:  ad hominem, illogical, flawed and inconsistent assertions.  No one has the ability to test every scientific theory.  Scientists, just like religious believers, place their faith in people, publications and other sources they trust, based on logic and experience.  As a believer in God, I encourage you to read and understand the writings of past atheists such as Nietzsche, Freud and Marx, as well as the new atheists, like Daniel Dennett, Sam Harris, Richard Dawkins and Christopher Hitchens.  They have a faith of their own, even though they are loath to call it that.  When you hear their ideas, you will see some of them are accurate, but some are flawed, just as the ideas of many religious individuals.