An interesting dilemma.
The problem here, I think, is that the concept of "citizenship" -- as is being referenced in regards to our being "citizens of Earth" -- implies something that goes beyond a sense of global siblinghood and instead suggests something rather more rigid. Citizenship is a concept that embodies and codifies our innate, abstract sense of membership in a community, the expectations we place on ourselves and on others (ideally seeking a balance between benefit to ourselves and contribution to the betterment of the rest of the group). Citizenship is a concept that builds on those natural social considerations, creating a system of artificial rights and responsibilities, which are supposed to make strong and clear the underlying ethical expectations but in fact (in my opinion) tend to obscure them. It makes them vulnerable to manipulation, by making the vessel more important than what the vessel contains. All things removed from the Unspoken become a matter of
politics rather than
ethics, and the tribal psychology of most humans makes the urge to engage in political manoeuvring difficult to resist, or so I've observed. There's an old metaphor about a hypothetical religion that establishes itself in devotion and service to a god - but over time it becomes so draped in ritual and ceremony, in grand architecture and riches, in social systems and hierarchies and politics, that the god withers away and dies, unmourned, while the people remain convinced that they're doing the god honour. That temple is rich and thriving, but it's spiritually empty. In the same way, every initially virtuous political concept becomes -- or can become -- just another hollow means of gaining power or scoring points, or arranging things to be the way you want them to be, regardless of the effect on others. Because ethics is replaced by politics, insidiously, very often without the people in question realizing it. And the worst part of all this, is that those who can see that the god for whom the temple was built is dead -- well, it kills their faith in god itself, so to speak. They see the rituals are meaningless, and in their bitterness they forget that the god ever meant something in the first place.
Keep talking about "rights" as shorthand for "something I've decided I want", for example, and the word no longer means anything; the dignity of the sapient being that the concept of a "right" was supposed to point to, supposed to denote, is drowned, lost beneath the understanding that political ideology and power struggle is all that matters. The devil can quote scripture for his own purpose -- raise a young person in a society where the devil is allowed to shout down anyone who speaks against him with twisted appeals to scripture, and that child won't think scripture is worth a damn. If you see my meaning. I would bet that anyone reading this can think of at least some politically charged words that don't mean anything to them anymore, because they've heard them too often, applied all over the place to anything and everything as a means of promoting certain ideological positions and vilifying others.
What I'm getting at here, OP (and yes there is a point to all this), is that good intentions can lead to bad places if the motives and desires and instincts of all those involved aren't questioned carefully. We must all meditate on these issues at length if we want to avoid not only turning the dream to a nightmare but destroying our capacity to place hope in dreams to begin with. Here, with the idea of world government, there's confusion as to whether those promoting such a thing want to transcend the notions of thoughtless nationality and narrow-mindedness - create a truly "better" future - or merely want to simplify the process of encouraging these notions, streamlining them, and in doing so allowing the worst excesses and abuses of the power-hungry and the controlling to lock into place over all of us. Whose world government will it be? "The people's?" Or a small group of entrenched leaders, two thirds of them well-meaning but dangerously blind ideologues, the remainder manipulative sociopaths? Humans on the whole aren't very good at noticing sociopaths, because they're naturally empathic and cooperative creatures who instinctively display incredible amounts of trust when you really think about it. More dangerous than those people, though, are the hoards of well-meaning types who buy into worship of that hypothetical god, and without realizing it fall into the trap of promoting the rituals and hierarchies without noticing that the god is dead.
Someone upthread pointed out that world government is to some people synonymous with tyranny, or at least the strong suspicion of such. The problem is that governments, by their very nature, seek to grow and to expand, to justify themselves by becoming ever more invasive and controlling. As Washington said, government is like fire - a dangerous tool and a fearful master. Of course, fire is one of the most useful, basic and necessary tools of the human species; hardly anyone with a brain would suggest we give up fire, or declare fire BAD. But it must be dealt with and utilized with caution. Don't get carried away and set things alight to see if they burn. And understand that fire spreads at an alarming rate, and that the fire which warms the house and saves you from the cold today can all too swiftly burn that house to the ground tomorrow. Unless you're responsible and careful. A government can turn rotten, especially as it gets larger and heavier. In the past, ideas from outside, the influence of alternative lifestyles and philosophies, has challenged the worst of those regimes. Sometimes only armed opposition has turned back destructive ideologies that have gripped whole societies. One world government will be something that potentially makes escape particularly difficult if and when the rot sets in and those talons begin gripping tighter than is healthy.
Given the urge towards ideological conformity, which can be observed in any number of social groupings, of any size, once people define themselves in terms of membership in the group, then a global society consisting of one national identity, rather than freeing us from the troubles that plague our world today in terms of oppression, mismanagement, corruption and ideological extremism, is probably more likely to stagnate us. It might well encourage a conformity of thought that will be resistant to any form of change. Or so some would say. Whole societies of millions of people can become gripped in fearful extremism; who's to say a population of billions couldn't do the same? Wouldn't feel he psychological pressure to do the same? Wouldn't be encouraged by the powerful to do the same? And in such a scenario there would exist no outsiders to offer alternatives or challenges. Instead of other cultures who offer new perspectives, would we merely have rebels and criminals, who dare to go against the Way?
Think of biology; the genetic roulette that results when females select from among a population of males and join their two genomes together, rather than simply spawning clones and waiting for the odd beneficial mutation to come along (which works well enough for some animals). The rapid pace of adaptation that results from that genetic shifting means that a two-sexed species placed in competition with a single-sexed species will out-compete it, and species only tend to revert to single-sex if they've found a really stable ecological niche. Do we want Earth to be a single-minded culture?
Your post spoke of our being "absorbed" into a world government, and that's essentially the core of the problem. What looks to be the Federation can so easily become the Dominion -
join us, become part of our peace, submit to our control and our ways. Independence from us is to be frowned upon, as those who seek it are a threat to us, to our safety and our stability. Control is a necessity. Means and methods of living and providing for yourself and others that keep you from dependence on us are wrong, and will be disallowed. You have been invited to join us, and join you will, one way or the other.
Don't get me wrong here; I'm not for a moment suggesting your angle here is
anything other than a desire for mutually beneficial unity, and I
strongly support any effort to encourage a sense of a global family, of a people who understand themselves to be fragments of the whole, drops in the ocean, unrestrained by division. Unity - in the form of IDIC - is a dream I share with you. But I am wary, ever wary, of the motives of some of the people who seek to crowd us all together. Do not assume that everyone who shares your stated goals shares your perspective, or your humanitarian concerns - and don't assume that those who genuinely
think they're honouring the god by proudly performing the rituals and ceremonies are actually serving that deity in practice.
Essentially, I would fully support the attitude that we are one people. One who is many. One global community of ever-shifting nations, religions, philosophies, families, etc. Above all, individuals, cooperating for mutual benefit, rather than cogs in the machinery of nation or party. Warfare, violence, hatred, oppression; these I hope can be ended as a means by which individuals and groups relate to one another, and towards that goal many of us work however we can. I am not, however, convinced that a world government as most people propose it will be anything other than tyrannical, if not initially then inevitably
in time, and probably without the majority being willing to acknowledge it, not through malice but through a combination of fear and simple psychological blindness. Going against the group is hard for most people once they've committed to membership in, or loyalty to, that group identity. So many times I've seen it: people associate with a certain government, nation, religion, creed, ideology, political party, etc and become blind to its failings and abuses while condemning such elsewhere. Hypocrisy. Double-think. Well-meaning inability to accept that the in-group is not better than any outgroup, or is (or is becoming) the very thing it claims to oppose. Blind spots.
Consider a series of experiments reported by Dawkins, in which a group of children were given historic accounts of pillage and mass murder and asked if they believed the people responsible had done wrong, and if they had, why. The answers were consistent, the children condemning the actions on humanitarian grounds...except when the example given had members of their own tribal religious nation in the role of the murderers and invaders. Dawkins being Dawkins, he used this to illustrate his belief in the blindness of religion, but really it was the blindness of
any affiliation, any political allegiance (he seemed to overlook the fact that he was dealing with a
tribal nation as well as a religious philosophy, and the former seems the more likely suspect by far). I would cautiously suggest: be wary of those who associate their ethical and philosophical desires with a political body or a political solution - you're building the temple; do not forget that it's the god that matters. Other than that, good luck in encouraging our global unity, and that's a sincere statement.
I hope my rambling here made *some* sort of sense.

That is, of course, just my opinion and musing on the subject.