I chose the title for my sermon today several months ago, when I was reading letters from our first Nobel Prize winner to his fellow poet, Dorothy Wellesley. (I have as yet no text in the conventional pulpit sense.) Since then, much has changed in the world around us, and it might seem that there are more pressing topics than a poet's private correspondence. Yet I want to persevere, hoping that urgent and reflective concerns can serve to illuminate each other.

In December 1936, little over two years before his death, W. B. Yeats was preoccupied with various corruptions of public life, affecting the practice of law and religion as well as politics. He consoled himself - or so he told his English correspondent - by remembering Jonathan Swift. Thinking more specifically of Swift as their fellow-poet, he described the dean of this cathedral as 'our ancestor'. He took comfort in the precedent that, in death, Swift had gone where 'fierce indignation can lacerate his heart no more.' Those English words of Yeats's translate the Latin epigraph which you can see here within these walls, on Swift's tomb.

What did Yeats mean by claiming Swift as an ancestor? Clearly, he did not have in mind any biological descent. Quite apart from the unremarkable pedigree which Yeats scholars can trace back into the seventeenth century, the topic of Jonathan Swift is notable for the rigour with which questions of marriage and paternity had been interrogated, a preoccupation which persists today. Swift, we may be sure, never married, and had no child whether legitimate or bastard. On this level, he is nobody's ancestor.

Yeats's claim, then, must be understood in another sense. Among the scandals mentioned in his letter to Dorothy Wellesley was a case - which I have not identified - in which a bishop was urging a certain man to take Holy Communion, though the man himself protested that he was not spiritually fit. Seen in this context, we may easily conclude that Yeats regarded Swift as a spiritual ancestor, and went further to propose that the Dean stood in the same relation to Dorothy Wellesley.

All would now seem to be well. Indeed, the brevity of these remarks may suggest that you have escaped the censure Dean Swift heaped upon those who, in his day, fell asleep here in his cathedral. 'Opium', he observed, 'is not so stupefying to many persons as an afternoon sermon.' On that occasion, Swift chose for his text a passage from the Acts of the Apostles where it is related that:

There sat in the window a certain young man named Eutychus, being fallen into a deep sleep; and as Paul was long preaching, he sunk down with sleep, and fell down from the third loft, and was taken up dead. (Acts 20:9)

If I am not yet finished preaching, I can nonetheless assure all those in the third loft that I will not detain them so long that they need fasten their safety belts. Yet there are things still to be said about Swift as ancestor this afternoon. While it clear that Yeats intended no literal sense of the word, simply to say that he regarded the dean as a spiritual ancestor is not enough.

Yeats had spent much time working on a play about Jonathan Swift, a play about - to be precise - Jonathan Swift in Death. With a séance as its deliberately bathetic setting, 'The Words Upon the Window-Pane' conveys to us what Yeats thought Swift's thoughts about parentage and (thus) ancestry had been.

There are several dramatic instances, highly surprising in the 1930s Dublin rooming-house where the action takes place. The mediumistic Mrs Henderson relays to her little audience words which Swift utters to the two most important women in his life. First, we hear him address Hester Vanhomrigh:

'I have something in my blood that no child must inherit. I have constant attacks of dizziness; I pretend they come from a surfeit of fruit when I was a child....'

Then, a little later, the voice of Swift is speaking less harshly to Esther Johnson:

'Are you unhappy? You have no children, you have no lover, you have no husband. A cross and ageing man for friend...'

Within the play, a character named John Corbet provides a running academic commentary on these disembodied messages from the long-dead, though as a sceptic he prefers to believe that Mrs Henderson is making it all up. Corbet suggests Swift may have refused to beget children - refused to be an ancestor, we might say - because he foresaw the collapse of monarchical government, foresaw the French Revolution and 'the ruin to come'. Mrs Henderson doesn't understand this high-falutin' stuff, and eventually she is left alone in the shabby drawing-room. The final moments of Yeats's play deliver the most startling example of Swift on the topic of ancestry. The medium potters round looking for a tea-cup and saucer. Alone and unobserved she is re-possessed by the voice of the Dean paraphrasing the Book of Job - 'Perish the day on which I was born.'

Here indeed is a lurid spectrum of roles or attitudes towards parentage and ancestry. As Yeats presents things, Swift is an angry but merely surrogate father to Hester Vanhomrigh, an abashed non-husband to the childless Esther Johnson, and finally a minister of the gospel cursing his own birth through misquotation of the Bible. Yet it is this same Swift whom Yeats advances as his ancestor.

Instead of pursuing the literary and dramatic aspects of the strange relationship between poets separated by two centuries, I would like to offer some comments on an ecclesiastical issue, even while I must admit a far lesser authority in so doing.

Jonathan Swift had been ambitious as a clergyman. On appointment to the deanery here, he recognized that his hopes of a bishopric had been virtually extinguished. That is to say, he would never stand in apostolic succession through the 'laying on of hands' by another bishop already in the apostolic succession. Political events in 1726 and 1727 confirmed his fears. Though it has been customary to play down the extent to which Swift was a fervent Christian, it can hardly be denied that he was aware of this terminus ad quem in his life as a churchman. Many deans subsequently occupied the throne of bishop or archbishop, but Swift knew that he was not to be of their number. Taking succession as a prelate to be a form of lineage, we can see that his appointment as dean put an end to his ever being the consecrator of a fellow-bishop. He would be no prelate's ancestor in an ecclesiastical lineage just as he would be (for other reasons, of which we learned but little) no biological ancestor either. It was a double denial, in part a self-denial.

Dozens - indeed thousands - of churchmen found themselves in the same situation. In churches where priestly celibacy is maintained, it is the rule not the exception. Even in the eighteenth-century Church of Ireland, fellowship of Trinity College required celibacy - in theory at least. This concept or convention of a successive lineage which is to be foresworn or sacrificed for a spiritual benefit plays a profoundly ambiguous role in Yeats's view of Swift as his ancestor.

But what about Swift as Dorothy Wellesley's ancestor? Here we must look hard at political issues, including the urgent ones. Dorothy married Gerald Wellesley in 1914. In 1943 she became Duchess of Wellington, for her husband had as ancestor Arthur Wellesley, the Dubliner who defeated Napoleon at Waterloo. The first Duke of Wellington was seen in his day as the successor to the first Duke of Marlborough, they were generals who had saved England from France. Here, however, we must pay close attention to an aspect of the Dean which Yeats busily suppressed - his hatred of warfare, and his especial contempt for John Churchill, the victor of Ramillies (as every school-boy pronounces it), Oudenarde and Malplaquet.

Swift's opinion of Marlborough is neatly expressed in 'A Satirical Elegy on a Late Famous General':

Behold his funeral appears,
Nor widow's sighs, nor orphan's tears,
Wont at such times each heart to pierce,
Attend the progress of his hearse.

But what of that? His friends may say
He had those honours in his day.
True to his profit and his pride,
He made them weep before he died.

This frank detestation of warfare, of professional generalship, of the alchemical transformation of other men's blood into revenue and estate, is part of any inheritance one would take on in treating Swift as a spiritual ancestor, today.

I would like to associate it with a more spiritual, if ambivalent, aspect of the dean's thought. I mean his attitude towards Church doctrine. In notes only published after his death, he recalled that:

The Christian religion, in the most early times, was proposed to the Jews and Heathens, without the article of Christ's divinity; which, I remember, Erasmus accounts for, by it's being too strong a meat for babes. Perhaps if it were now softened by the Chinese missionaries, the conversion of those infidels would be less difficult: and we find by the Alcoran, it is the great stumbling block of the Mohametans.

Swift's concern here with missionary matters was not characteristic. And his published views on Islam were of a more satirical kind. Yet, in the present state of the world, we might do well to dwell on this obscure jotting. When Yeats in the 1930s was keen to re-create Swift as a tragic figure, he employed notions of ancestry and descent, authoritarian government and fear of the future. Today it may be possible to discern a less melodramatic yet tragic figure. In this tentative interpretation, I find Swift perhaps suggesting that, in order to honour the Sermon on the Mount, in order to say efficaciously and with conviction 'Blessed are the peace-makers', it may be necessary to avoid an insistence on Christ's divinity.

We have, in other words, reverted to a state of global barbarism not unlike that which obtained during those 'most early times' of Christendom to which Swift alludes. I happily concede that, elsewhere and always, Swift is a thorough-going Trinitarian in his expressions. But, for a moment at least, he muses on a form of Christian tragedy which we might attend to with spiritual profit, as Crusader politics and the so-called 'doctrine' of pre-emptive warfare sow the seeds of extended religious conflict.

In honouring Jonathan Swift each year in his cathedral, we are obliged to look at the difficult and anomalous parts of his legacy. These are uncomfortable times and uncomfortable thoughts may be more fitting than celebration and vain repetitions of the familiar. So, I shall conclude, instead of starting, with my text. It is chosen from a little used source, indeed an Islamic one and indeed a secular one. Rahim Abdul Karim, a retired Iraqi school-teacher, said of the forces claiming control of his country, 'They are killing us to save us.' I suggest that, in his words, we hear a true echo of Swift's irony and compassion.