Darryl Hart has raised some outstanding points with regard to the nature of the language of the Manhattan Declaration. You can read them here. Here is one such helpful point:
My last and biggest reservation is related to the Social Gospel aspects of the Declaration â€“ that is, the idea that Christianity leads to and promotes a just society. Donâ€™t get me wrong. I donâ€™t want to be heard to be saying that Christianity promotes injustice, though, of course, Christianityâ€™s record in human history has not been free from embracing tyranny and injustice (at least as defined by the likes of Kant). But do the authors of the Declaration believe that Jesus and the apostles would have signed a Rome Declaration if one were available to them? In other words, is the purpose of Christianity to progress this world or is it to prepare believers for the next? Is the purpose of the gospel to yield the common good or eternal salvation? I understand that Protestants and Roman Catholics (I have interacted less with Orthodox about this) differ on questions of continuity and discontinuity between temporal and eternal goods. Will truth and justice and prosperity in this world be like the truth and justice and prosperity that believers will experience in the new heavens and new earth?
If it is legitimate to raise this question, then the Manhattan Declaration needs to address the concerns of those Christians who believe that the gospel has a higher aim than simply the right ordering of this world. This doesnâ€™t mean that necessarily that the Christianity of which I speak is opposed in fundamentalist, docetist, or gnostic fashion to a good society, or to ordered liberty. But I do worry that by directing so much attention in the name of Christ to the great moral concerns of this age, Christians will lose sight of the eternal truths that older professions of the church recognized (and encourage non-Christians to look to the church for solutions to societyâ€™s problems. Older expressions of Christianity put the problems and even the evils of this life into a perspective that saw them as not ultimate but temporary.