I've thought about this a lot, particularly in college when I was taking classes about the history of religion in the United States. Let's see if I can articulate myself here.
There are so many situations in which religious values dictate moral and social values. Abortion is the esiest and most obvious example. While I won't deny that there are non-religious people who do not believe that abortion should be legal, the vast majority, it seems, use a religious argument. Those Christians who argue against abortion generally say that it is the equivalent of murder because we are ending the existance of a living thing that
is already a human being. The Catholic Church believes that life begins at conception, and many conservative or evangelical denominations agree (although let's not forget that 30% of evangelical Christians are liberal...). Traditionally, Hindusiam and Islam also prohibit abortion is a crime agains nature and a form of murder. In addition, many denominations within Orthodox Judiasm prohibit abortion after 40 days and strongly caution against it before. Of course, most liberal denominations of Judaism acknowledge that there are situations in which abortion is necessary, and many denominations of Christianity agree. I would also be remiss in not mentioning that there are many people who do come to the conclusion that their religious beliefs should not affect legislation that dictates the freedoms of all citizens of the United States, regardless of religion or beliefs.
This is why secular arguments are so important. How is it fair to say: "my Bible states that G-d sees abortion as a crime against His creations, therefore, no one can have an abortion, even if their religious beliefs state differently than mine"? This makes little sense, and, it can be argued, directly violates the First Amendment (I will follow up on this later). There are secular arguments for and agains abortion, and these are more compelling. Acknowledging that all Americans have the right to "Life, Liberty, and the Pursuit of Happiness" is one argument, because life is mentioned as one of our basic rights. But then the argument returns to when a zygote becomes a fetus and when that can be considered human life...or the potential for human life. Unfortunately, many arguments base this on religious beliefs, with one group arguing that life begins at conception, another that it begins at 22 weeks, and still another that life begins at birth. This gets us nowhere. I would argue for using science, which can determine the approximate time when the bran forms the capacity for human pain, emotion, and thought.
Now for the First Amendment, which states "Congress shall make no law concerning the establishment of a religion, or prohibiting the free exercise thereof." Now, if my religion tells me that abortions are acceptable in certain situations and someone was to tell me that I couldn't have one because
their religion prohibits abortion, wouldn't that person be forcing her religious beliefs on me and be preventing me from freely exercising my religion? I can see how someone might argue for the reverse, that allowing abortions to remain legal would violate the religious beliefs of those who do not believe in abortion, but this simply isn't true; no one is forced
to have an abortion, so this person would not be forced to do something prohibited by her religion. By making abortions illegal, those who believe that abortion is acceptable in some situations
would be prohibited from acting in accordance with their beliefs, religious or otherwise.
I do understand that, for those of us who are religious people, many opinions and beliefs are shaped by our religion's dictates and standards. We cannot escape this. What we need to be careful of is forcing those beliefs on others. If you carefully read the Constitution, it does not prohibit us from allowing religious beliefs to affect our secular lives, but rather urges us to also respect the religious beliefs of all Americans.