There are–I think–several reasons why America’s general attitude towards religion is different from Europe’s.
I guess the first one is that America is relatively recent in terms of the history of the world. This doesn’t mean that people from the US have committed less mistakes but more importantly that the consequences of their basic insights, which usually only emerge in the long term, are not necessarily visible now. Among those “basic insights,” religious convictions must be included. If the consequences are not tangible, people are less worried about being upfront in presenting their faith, or lack of.
Secondly, religious conflict has not been the rule in the history of the US. Europe keeps the bitter scars of religious persecution, mainly between Catholics and Protestants, but also between Christians and Muslims, or between Jews and Christians. The open contradiction between the message of love that religion purportedly has, and the reality of so much cruelty has fed and strengthened the seculaist / humanist cause since the times of the Reformation, or even before.
In the third place, evangelical preaching marks a big difference as for the public opinion towards religion. Protestantism is the version of Christianity that arrived to the US. With the precedent of cutting off centuries of Papal authority, it wasn’t too difficult for Protestants to slip into successive splittings that gave origin to hundreds of Christian denominations, small or big, more traditional or more progressive, etc. Hence a mindset of competition was born. Potential parishioners were then regarded as “customers” that have to be given reasons, should you want to keep them attending your church and giving you their alms. Evangelical preaching targeted therefore the individual with his more immediate, existential and practical concerns. Those who got workable answers flowing from the Bible would never regard the faith as something that impoverishes their lives or humiliates their intellect.
Fourthly, many great Americans were and are unabashed of their faith. Almost every president has made reverent reference to God as source of blessing for his people, and this has been generally regarded as normal and praiseworthy. In America’s outset, the Pilgrims celebrated God’s Providence. It is a hard case, as well, to prove that those who signed the Act of Independence were Deists (let alone atheists) for the references to a personal God are not scarce in their writings. Faith, and in particular Christian faith, has been a tremendous source of inspiration for many, so that in America it is the Humanist who has to prove that the Jesus’ teachings and example and promise of hope diminish the true human desires and longings.
Lastly, we all know that America is almost idolatrous when it comes to the notion of newness. Beyond the light spirit that takes delectation in demolishing not-that-old-buildings to leave room for “brand-new” ones, there is in America a true appreciation for the promises of the future, even to the point of not valuing sometimes all the riches of the past. This mentality springs from a religious well. Science has proved to be morally neutral, equally efficient in creating better conditions for life or destroying every trace of dignity in the human race. Of course, religious people can be good or wicked but the distinct message of religion will be the first in line to accuse any perversion of its own character. In that sense, a nation who cherishes hope hardly can part company with the deepest and most abundant source of hope.