A threat of a different lifestyle. There is also a component of fear, even jealousy. Traditional, conservative, fundamentalist, & dogmatic religionists tend to look upon atheists w/suspicion because they contend that their purview is the only legitimate one. These religionists contend that everyone, w/o exception, should believe as they do. They exemplify prejudice to the utmost degree. They are so ensconced in their belief system that they cannot, do not, &/or will not see a different respective ethically or spiritually. They are so arrogant to believe that they have the right path to God or a Universal Being. They are also parochial in their religious perspective.
These religionists are threatened by atheists in that atheists are different. Also atheists do not live their lives in strict, dichotomous prescriptions like they do. Atheists are free to question religious perspectives & live life as THEY please as opposed to these religionists who must always submit to SOMEONE ELSE'S religious prescriptions. They are envious of the atheists in that the latter are living FULL, JUICY, & RED lives as opposed to them who are living FEARFUL, even GRAY lives of utter resignation. Atheists are doing & living what they secretly wish they could live but are FEARFUL of doing so lest they will be judged & consigned to hell. In other words, atheists are IN CONTROL while these religionists ....ARE CONTROLLED.