JOHN WATERS wrote tellingly in The Irish Times last Friday that he would personally find it difficult to discuss transubstantiation with his best friend, "not because I have problems with the doctrine but because such matters are impossible to discuss in the language we use for politics, shopping and sex".

His article carried the excellent headline: "Belief in transubstantiation not a matter of yes or no". In view of the celebration of the Eucharistic Congress, it is timely to consider for a moment a term that might be used unthinkingly and perhaps unwisely during the congress.

When Richard Dawkins, the well-known evangelical atheist, instructs Catholics on the terms of their membership of their own church, it is time to sit up and take notice.

Speaking in Dublin last week, he remarked in his familiar and peremptory fashion: "If they don't believe in transubstantiation then they are not Roman Catholics" (The Irish Times last Thursday). So there, now you know: straight from the alternative magisterium.

Hardline conservative Catholics may be delighted to have such a distinguished ally. On further thought, however, they may be embarrassed to receive support from such a quarter.

One notices that Prof Dawkins, in his eagerness to discredit all religious belief, often sails into his task with an unwise disregard for the meaning and origin of a technical term like "transubstantiation", which is not to be found in the Bible, and was first used in the 11th century.

Roman Catholics believe in the real presence of Jesus Christ in the Eucharist. "Real presence" is not synonymous with "transubstantiation" which would never have entered liturgical life had it not been seemingly prescribed by the Council of Trent in the 16th century.

In that age of religious strife, the term was used to differentiate aggressively between Roman Catholics and Protestants. In our own more ecumenical age, we try to dwell on what unites rather than divides us.

Lest heresy-hunters claim that I am not in conformity with church teaching, I quote the relevant words of the council: "By the consecration of the bread and wine there takes place a change of the whole substance of the bread into the substance of the body of Christ our Lord and of the whole substance of the wine into the substance of his blood.

"This change the holy Catholic Church has fittingly and properly called transubstantiation."

Consequently if we use the language of substance and accidents in treating of the Eucharist, we must follow the teaching of the Council of Trent, which simply states that the term "transubstantiation" was fittingly used by the church.

Since, however, the church does not impose any philosophical system in the name of faith, we are under no obligation to use the term "transubstantiation" in discussion of the Eucharist.

Pope John XXIII, in his opening address to the second Vatican Council, made a hugely important distinction between the substance of faith and the manner of its formulation. Prof Dawkins cannot be expected to be aware of this distinction or indeed to appreciate its significance – but if he wishes to pontificate on the criteria for membership of the Roman Catholic Church, he might profitably do some study of critical theology.

Much ecumenical progress has been made on the Eucharist. It is illuminating to conclude with a short quotation from the magnificent Lima Statement of 1982, when more than 100 theologians from many churches in the world, Protestant and Catholic, met in Lima, Peru, and wrote an agreed document on the subjects of baptism, Eucharist and ministry which they sent for study to all their churches.

From the statement on the Eucharist, these few words go to the heart of what the Eucharist means: "The Eucharist thus signifies what the world is to become: an offering and hymn of praise to the Creator, a universal communion in the body of Christ, a kingdom of justice, love and peace in the Holy Spirit."

Fr Gabriel Daly is an Augustinian