He described "the triple jeopardy of aggressive secularism, radical Islam and a misplaced understanding of multiculturalism".
O'Brien said aggressive secularism threatened the Christian heritage and he wanted religion to remain in the public sphere.
But what defines Western Europe - as well as the American left - is its aggressive secularism.
Their movements had often evolved in a dialectical relationship with an aggressive secularism which showed scant respect for religion and its adherents.
The amendment is necessary, its supporters say, to protect believers against aggressive secularism in the schools and courts.
Pope Benedict used his recent trip to Britain to condemn "atheist extremism" and "aggressive secularism".
He has singled out the spread of "aggressive secularism," especially in Europe and North America.
"Most of Israel seems to be in the grip of a relentless and aggressive secularism," he said.
Stanford professor Van Harvey refers to this as a "background of aggressive secularism and the almost complete neglect of the academic study of religion".
A number of speakers spoke on how what it terms "aggressive secularism" posed a threat to rights of conscience.