The liberal, secularising wings of both Protestantism and Judaism were drawn to this position. Felix Adler was a prominent Jewish proponent of the idea, writing in 1878 that Jews had a special role in pushing along a process of universalisation until the point was reached in which their own "distinctiveness will fade. And eventually, the Jewish race will die."
Not all liberal Jews accepted Adler's position. Mordecai Kaplan argued,
that Jews did not need to be justified as the people chosen by God for the sake of a unique monotheistic mission. Every nation and culture had the right to perpetuate itself, albeit without harming others.
Kaplan was not a conservative, but his position is the one held by traditionalists. On reading the early chapters of Kaufmann's book, I was struck by how "overloaded" the idea of Anglo-Saxonism was in the U.S. It was as if Anglo-Saxonism was valued not in itself, but in its "mission" or "destiny" in bringing a liberal order to a new continent.
An ethnic tradition should be valued, amongst other things, for its unique character and culture; for its contributions to the arts and sciences; and for its significance to the identity and sense of belonging of the individual. It doesn't need to be exceptional in its global mission.
I've had a quick look at Adler's beliefs. A few things stand out. Adler drew from a range of intellectual traditions: he was raised within Reform Judaism but was rejected as a rabbi for his lack of orthodoxy; he drew also from the philosophy of Kant and the transcendentalism of Ralph Waldo Emerson.
Emerson seems to have been particularly important:
While morality had been an oppressive element in the Christian tradition...for Emerson our moral sense makes us free. Moral sensitivity enables us to become the architects and sculptors of an autonomous personhood. It was this thought that ethics can be creative and reconstructive that entranced the young Felix Adler and set him on the path that led to the development of Ethical Culture.
We still have here a core belief in "freedom as autonomy," alongside a focus on the "creative spirit" aspect of human nature - a drive to "reconstruct" the self and the society we inhabit.
Adler himself wrote:
And this is the prerogative of man, that he need not blindly follow the law of his being, but that he is himself the author of the moral law, and creates it even in acting it out.
This, at the very least, sounds like the modern liberal idea that it is autonomy (self-authorship) that gives man his dignity. Adler went on to write:
We are all soldiers in the great army of mankind, battling in the cause of moral freedom.
Battling not for moral goodness but for moral freedom? To author and to create our own moral law?
I would need to study Adler's writings in greater depth to make a more certain comment here, but from what I've read Adler went through a crisis of faith that struck many intellectuals at the time and he seems to have responded by setting up "Humanity" as a new god - in this literal sense he was a liberal humanist.
It makes sense if this is the entity you seek to serve that you might then become a globalist or cosmopolitan. If you are serving a larger humanity, as a replacement for God, then you won't want people to look to their own particular communities - you will think it to be progress if people give up "parochial" identities in favour of a single global one of a common "Humanity".
(Another thought occurs to me: if you are aiming at "moral freedom" then you might find renouncing particular allegiances an appealing move, as a duty to humanity in general is much more open and non-specific that the particular duties we have to family, tribe, church and nation.)
Traditionalists do not think it moral to renounce particular communities and identities in favour of a single global one. Our closer relationships and identities do have an important claim on us and it is moral for us to discharge our duties to each one, beginning with self and family, and running on to community, ethny and nation, and then finally to a common humanity.