If the researcher were to ask me if I believe in God and had to answer yes or no, I couldn't do it. I'd have to say: if you mean by God some outside entity beyond nature that can do things to the world and humanity, no. But I am open to mystery and have faith in love, so if you mean by God "Mystery" or "Love," yes, I believe in God.
To judge the religiosity of America, I think researchers need to ask what is a person's understanding or image of God. And why they go to church or synagogue or temple or mosque, if they do. Also they need to take into account that some people who do not attend religious services or belong to a religion can believe in a "Great Person out there" and be quite religious.
In judging US religiosity, e.g. compared with France or China, I would urge a question on what a person means if she says she believes or does not believe in God. For example:
What approximates best your understanding or image of God.
- Creator and/or Sustainer of the world and humanity.
- Divine parent (heavenly father, nourishing mother).
- Great or Holy Spirit (outside matter).
- Supernatural being or super human personal entity involved in human life.
- Judge, Rewarder, Punisher.
6. First Cause that is itself uncaused,
7. Principle or Ground of all being,
8. Being of all beings,
9. Reason, Nature, or Universe
10. Absolute, first beginning and last end of all things.
14. Integral Spirit
15. Universal Consciousness
1-5 might be called the Gods of Religion (Personal Entity); 6-10 the Gods of the Philosophers (Rational Principle); 11-15 the Gods of the Contemplatives and Mystics (Experience of Wholeness).
In one sense, everyone is religious in the sense that they are part of a culture with a specific language, memories, rituals, and celebrations. In this sense religion is identified with culture or at least a universal aspect of culture. In that sense no nation or group is more religious than another.
But in the traditional sense of religious as opposed to secular or contrasting religious belief with rational explanation, I would argue that only those who say they believe in God and whose image of God approximates 1-5 are religious in the traditional sense. (Conversely we might infer that those who say they do not believe in God and whose image of God is 1 to 5, might indeed be believers of God in another sense.)
I would further argue that only those who identify themselves as belonging to some religion because of firm and constant adherence to that religion's belief system (creed, scriptures, organizational rules) should be considered religious in the traditional sense whether or not they attend religious services. Those who identify with a people socially or ethnically, but do not accept the creed, organizational rules, or scriptures as divinely instituted, should not be considered religious even if they participate in a religion's services, celebrations, memories.
So I suspect that the majority of Americans, though church-going and believing in God, are not religious. And combining them with persons who claim to be atheists or nonbelievers, religiosity in America is not as pervasive as social commenters say, much to the chagrin of those who are religious and think that the rest of us are bringing judgment and damnation on us all.