Can personal values be detached from Christian belief? UPDATED

There is a passage in Romano Guardini's book The End of the Modern World where the author suggests that, when the sense of the personal value of human existence is separated from its roots in Revelation, though that sense of personal value may survive for a time, it will eventually fade. The passage occurs in section V of the last chapter.

Personality is essential to man. This truth becomes clear, however, and can be affirmed only under the guidance of Revelation, which related man to a living , personal God, which makes him a son of God, which teaches the ordering of His Providence. When man fails to ground his personal perfection in Divine Revelation, he still retains an awareness of the individual as a rounded, dignified and creative human being. He can have no consciousness, however, of the real person who is the absolute ground of each man, an absolute ground of each man, an absolute ground superior to every psychological or cultural advantage or achievement. The knowledge of what it means to be a person is inextricably bound up with the faith of Christianity. An affirmation and a cultivation of the personal can endure for a time perhaps after Faith has been extinguished, but gradually they too will be lost.....

Guardini identifies a dishonesty on the part of modern man:

Modern man's dishonesty was rooted in his refusal to recognise Christianity's affirmation of the God-man relationship. Even as the modern world acclaimed the worth of personality and of an order of personal values, it did away with their guarantor, Christian Revelation.

It is interesting to read this in the context of Pope Benedict XVI's remarks about the relationship between religion and political life at Westminster Hall:

The central question at issue, then, is this: where is the ethical foundation for political choices to be found? The Catholic tradition maintains that the objective norms governing right action are accessible to reason, prescinding from the content of revelation. According to this understanding, the role of religion in political debate is not so much to supply these norms, as if they could not be known by non-believers – still less to propose concrete political solutions, which would lie altogether outside the competence of religion – but rather to help purify and shed light upon the application of reason to the discovery of objective moral principles.....

I would suggest that the world of reason and the world of faith – the world of secular rationality and the world of religious belief – need one another and should not be afraid to enter into a profound and ongoing dialogue, for the good of our civilization. 

It is also interesting to read, or listen to, the Christmas messages of the leaders of the three major political parties in the same context. These messages can be found here: Theresa May, Jeremy Corbyn and Vince Cable.

In the cases of Jeremy Corbyn and Vince Cable, one is led to wonder whether they are in fact Christmas messages at all. The separation of human values from Christian belief is absolute. Guardini's critique applies in all its rigour.

Theresa May fares better in maintaining a connection between our country's "Christian heritage" and the values that are shared across society as a whole. One wonders whether, speaking in a more strictly individual capacity rather than that of a party leader, she might have maintained that connection more strongly.

One can notice the difference between these messages and that of Her Majesty Queen Elizabeth II, with its explicit affirmation of her Christian inspiration (at 6:24 ff).

Pope Francis' Urbi et Orbi address also represents an example of human values that are firmly rooted in Christian revelation, with an affirmation of the incarnation of Christ preceding his survey of, and intercession for, regions of the world suffering from conflict:

Before the mystery of the Word made flesh, Christians in every place confess with the words of the Evangelist John: “We have beheld his glory, glory as of the only-begotten Son from the Father, full of grace and truth” (Jn 1:14).