Is today's Catholic really a Catholic? How do we define a Protestant? I ask the questions in the light of opinion polls which reveal not only a startling decline in religious belief and church-going among Catholics in the Republic, but also that many people in Northern Ireland do not wish to be termed Protestant or Catholic.
The wide-ranging Belfast Telegraph/LucidTalk poll surveyed more than 1,000 people here, but significantly only 60% said they were 'Protestant' or 'Catholic', 12.9% said they were 'other religion' and 26% said they had no religion.
Last week's Irish Times poll shows that many Catholics have given up on the basic tenets of the Church's teaching.
They no longer go to Mass and large numbers don't believe in transubstantiation - an essential doctrine of the Church.
Only 31% of Irish people go to Mass every week. Those who attend Mass regularly are twice as likely to live in rural, rather than urban, areas, where churches are facing their toughest crisis of confidence.
The findings underline the rapid decline of the Catholic Church, but I suspect that, if attitudes of Protestants had been surveyed, a similar trend would be highlighted.
The land of saints has, in fewer than 50 years, become a land of increasing secularism.
Our lives, Catholic and Protestant alike, have been determined by our religious differences. Now we find that fewer and fewer care about going to church, or about the fundamentals of belief which have split the peoples of Europe apart since the Reformation and contributed to the partition of this island nearly a century ago.
The Eucharistic Congress taking place in Dublin this week will undoubtedly afford many Catholics an opportunity to review and revive their faith and tens of thousands will take the opportunity to do so.
However, the reality is that the congress is taking place in a country which can no longer be described as 'devoutly Catholic', any more than Ulster can be termed 'staunchly Protestant'. These old cliches are losing their meaning.
This is not to deny the deep faith and continuing commitment of so many people to the Catholic Church or, for that matter, to Protestant dominations. Nor is it to suggest that Christian values are forsaken because people don't go to church.
The opinion polls pose wider questions for all of us - not least in Northern Ireland, where religious differences mean so much.
If you say you are a Protestant or a Catholic these days, what exactly does that mean? Is it just a convenient term to be used to ensure you remain different from your neighbour, or political opponent?
Or are you living out an illusion based on your upbringing? Are we confusing cultural differences with outdated religious affiliation?
For better or worse, the decline in worship represents the greatest social revolution on this island in our lifetimes. It may prove far more significant than the Good Friday Agreement, or any of the political steps taken so far to bring us closer together.
Increasingly, many so-called Catholics and Protestants don't go to church. They have little or no faith. They don't care whether their children have any, either.
The fact that they are all cut in the same secular mould raises major issues as to why so much of our society remains segregated.
The decline in belief and church attendance has huge implications for our future political direction, north and south of the border. We are already seeing this in Dublin, where politicians distanced themselves from the Catholic Church and Cardinal Sean Brady in relation to the child abuse scandal.
Who would have thought that an Irish government would close its embassy at the Vatican?
Here in Northern Ireland, the ties between politics and the churches remain strong. The SDLP and Sinn Fein are pan-Catholic parties, just as the DUP and Ulster Unionists are Protestant. Yet surely, as secularism takes hold, we should expect this stark politico-religious split to blur and all these parties to make more effort to open their doors and attract support from the wider community.
In terms of fair employment legislation, can we go on pigeon-holing people seeking jobs with simplistic Protestant and Catholic labels?
Is the question on the census form about religious affiliation not rendered meaningless if a majority of the public rarely - if ever - darkens a church door?
Perhaps the biggest challenge is segregated education and housing. In a society which is more secular than religious, is it not time that more of the walls dividing our schools and our homes began to fall?