Why are there so many Christians who want to deny free speech to their atheist brothers and sisters?

Christian American adults have less tolerance towards atheists than towards people who belong to a different Christian sect or a different faith. However, religious affiliations in America are changing rapidly. Between 1990 and 2001 the proportion of United States citizens without religious affiliations apparently rose from 8% to 14%, while adults believing in Christianity dropped by 1% per year during the same period. Should this this decline continue over a generation, the United States will change dramatically. Not all those without religious affiliations are Atheists, agnostics, freethinkers etc but many are. As a consequence, a “siege mentality” may be developing among the Christians, leading many to try and deny free speech to those who so persuasively reveal the myths of Christianity.