The reason the United States is still highly religious compared to Western and Northern Europe is because the United States is not joined at the hip with the church like in Europe. The Protestant Reformation was suppose to free the people from the power of the Catholic church, and allow us to create a more personal relationship with God. Yet, in those European countries that adopted Protestantism such as England for example, they established their own state-run churches that wielded political power just as the Catholic church had. Even non-members had to pay taxes to those state-run churches. And over time, those churches declined in membership and led to the rise in secularism in Western and Northern Europe.
No one wants to attend churches run by the state after all. The state corrupts religion by intertwining it with politics and you end up with political ideology wrapped in the guise of religion. It is the same reason that liberal denominations are on the decline in the United States, while evangelical denominations are on the rise. People attend churches, become followers of religion to be closer with God, and learn the text and understand His teachings. They do not go to hear someone lecture why it is Christian to promote social justice and champion gay marriage, things most Christians do not support anyways. People want true religion, and it is impossible to have when the state runs the churches and in a way sets the agenda.
It is the reason that we in the United States do not allow for the creation of any state-run church. The United States was founded in part because of religious freedom and religion has always played an important role in our country even today. The Founding Fathers wanted to keep religion pure and not force people of other churches and denominations to pay to one central church they may or may not agree with. We in the United States also have respect for those of other religious faiths because our country was founded by those of numerous faiths. Puritans in Massachusetts, Quakers in Pennsylvania, Catholics in Maryland, and Anglicans in the other colonies. We know how to live in peace with others whether they are Baptists, Catholics, Jews, Buddhists, Sikhs, you name it.
Yes the United States is a predominately Christian country and it always will be. In fact, last year, the growing number of non-religious individuals actually declined. As in a lower number of people in 2012 moved into the non-religious column as compared to the growing number that had preceded it. Our country was founded by men of various Christian beliefs and because we did not go the route of Europe and corrupt our churches, religion continues to be important to us. If you could go to Europe and abolish all of the state-run churches maybe more Europeans, Western and Northern Europeans that is, would be more religious. Not all of Europe is secular now. Poland is still a very Catholic and very devoutly religious county. Catholicism helped bring down atheist communism decades ago. And other European countries such as Romania, Italy, Portugal, Greece, and Ireland still have high religiosity rates too. The lowest rates are all in Protestant countries that adopted their own state-run churches, France being the exception.
And the United States as we know was founded on Judeo-Christian principles. Most European countries were not founded on religious principles like ours was, but through political upheavals.