If they have faith, then there is unfortunately nothing you can do to prevent them from being concerned for your soul. The best course of action you could take would be to downplay the situation and not make a fuss about it. If you're set on giving some sort of explanation, which I wouldn't recommend, you're better off resorting to a feel-good Carl Sagan or Neil deGrasse Tyson video than attempting to write a polemic against religion--Christopher Hitchens did a very good job at this, but his writings and appearances at debates did not bring about converts. Rather, it re-affirmed the convictions of both atheists and believers.