This used to be said of Catholicism.
And, at the time, it made considerable sense. The Catholicism of the 1800s was generally quite hostile to democracy, freedom of religion and other American values. Catholic Americans have been fully accepted by their Protestant countryment largely because American Catholicism changed and accepted these values. Assuming other religions do the same, I have no problem with them becoming equally integrated into our society.
After it's comportment during the dark ages, I can understand why. While I'm not a big fan of Catholicism by any means, I don't believe Catholicism stands on a par with the other religions I mentioned.