The Pope has continued with the argument he wants to have with British secularism at a joyous outdoor Mass attended by 70,000 in Bellahouston Park in Glasgow, Scotland -- reprising a phrase which has come to define his pontificate. 

The evangelization of culture is all the more important in our times, he said in his homily, "when a 'dictatorship of relativism' threatens to obscure the unchanging truth about man's nature, his destiny and the ultimate good."

He went on: "There are some who now seek to exclude religious belief from public discourse, to privatize it or even to paint it as a threat to equality and liberty," he said. "Yet religion is in fact a guarantee of authentic liberty and respect, leading us to look upon every person as a brother or sister."

He went on to call on lay people "not only to be examples of faith in public, but also to put the case for the promotion of faith's wisdom and vision in the public forum." Society today, he said, "needs clear voices which propose our right to live, not in a jungle of self-destructive and arbitrary freedoms, but in a society which works for the triel welfare of its citizens and offers them guidance and protection in the face of their weakness and fragility."

There were some moving moments in the Mass: stopping to kiss babies held up by security guards, and giving Communion to profoundly disabled people -- another way of giving the same message.

The mood among Catholics here is buoyant. The message is fierce, but the messenger is humble. Atheists and secularists are outraged; it's the end of Day One, but we already have what Catholics have long wanted: a national debate on the role of faith in public life.

Tomorrow begins at a university college in south-west London when Pope Benedict conducts a dialogue with educators, young people and leaders of faith. In the afternoon comes what is reckoned, from a political point of view, to be the heart of his visit -- an address in Westminster Hall, in the same room where St Thomas More was sentenced to death for refusing to accept the doctrine of the state.

It is a momentous place from which to make the philosophical case underpinning his opposition to the "closed secularism" chilling public life. I am fortunate to be among the 1,000 "civic leaders" -- including, of course, parliamentarians -- invited to hear him.

The new coalition Government has moved to identify itself with the Pope's call. The Conservative Party chairman, Baroness Warsi -- a Muslim -- gave a speech yesterday which was on the front page of the Telegraph. She says the Government "understands faith" and wants religious groups to play a greater role in British public life. Addressing Anglican bishops yesterday she said they would have "more power, more responsibility, and more choice", and that faith groups were "at the heart of society" and "key to its future". She criticised the previous Government's record, saying faith was treated as being a matter of "oddities, foreigners and minorities".

One can feel the ground shifting. Tomorrow will be fascinating.