I find Max Carter to be an excellent writer who, in his "On Faith" essay (http://newsweek.washingtonpost.com/onfaith/panelists/max_carter/2010/07/are_all_religions_the_same.html?referrer=emaillink) paints an appealing vision in which we all, whatever our religious backgrounds, are equally members of God's family, with God as father and mother, sharing the same DNA. Yes, I agree that we are all children of God, vessels made in the image of God, eikons of God, and I understand Max's emphasis on our commonalities.
However, I think the question Prothero is raising is not "Do we have the same spiritual DNA as humans?" but ,"Do all religions share the same spiritual DNA?" So I believe Carter is answering a different question.
I also see Carter putting himself in the God position rather than human position: He writes:
"Are all religions the same? Of course not. Nor are my three biological children [note the slide from religious institutions to individual children], even though they came from the same parents. While sharing many common characteristics, our two daughters and one son are as different as can be in many respects - religiously, politically, vocationally, temperamentally.
But my wife and I focus on our deep love of each, irrespective of those differences. We love no one of them more than the other; favor no one of them more than the other. We recognize the biological similarities, their common interests in living meaningful lives, their desire to be loving partners, and their devotion and love for us as parents. They have the same hopes and fears that any human beings have: anxiety about job security, health, their children's lives, the future....
I think of the world's religions in much the same way. "
In this analogy, Max puts himself in the parent/God position, looking down from above on his children/the world's religions. But we humans are not in the God position--we are not standing above the table of religion, looking down. As humans, we are at the table, constrained by time and place, looking across. And maybe because of that, our vision isn't so clear.
Thus, while I believe one can be a hyphenated universalist (and I am, in the traditional 17th century Quaker sense, a Christian-universalist), I think it is difficult to be a stand-alone Universalist, because, whether you mean to or not, that can easily slide into the false-for-a-human God position: "Yes, I am the parent and I love all religions equally. I am above the fray." But we are not. We are the fray.
At the end of his essay, Carter slides back into the "child" position. We can all be brothers and sisters, fighting and yes laughing, at the same picnic. It's a lovely picture, but I would argue we can't have it both ways: we can't be both parents and siblings.
I am passionate and perhaps a bit cranky about this subject, but I believe--along with people such as Marcus Borg--that we need to pick a faith and embrace it tightly, dig deeply into it, and acknowledge its humanity, including its deep flaws. We need to try to correct our faith's problems, but we can't do that if we don't have a faith. Further, we can't be "above" all faiths and then call our interfaith partners "brothers and sisters." We can't treat them with full respect as equals if we take the stance that we are the parents and they are the children. Or so I think.