Read the following paragraph and underline the topic sentence.
The most vital difference between Muslims in America and their brothers in Europe is the U.S. enduring emphasis on religious liberty. Religion is given far more respect in the U.S. than in Europe. Think about it. The U.S. is in the midst of a resentful debate over immigration in which many Americans reject “hyphenated （带连字符的） identifies” like Mexican-American as a threat to national unity. Yet while Protestant, Catholic and Jewish Americans may disagree strongly among themselves, the religious basis of their identity is not seriously questioned by anyone. If Muslim Americans are not so readily accepted today, it is not because they are believers. In Europe, by contrast, Muslims are resented and marginalized precisely because their religion threatens strong secular （世俗的） values.