Church doctrine and treading the line between development and heresy

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In Greek legend, Theseus, the King of Athens, rescued the children of Athens from King Minos of Crete after slaying the Minotaur. He then escaped from Crete on a ship heading to Delos. This ship was preserved at Athens and taken on an annual pilgrimage to Delos to honour Apollos.  
As time went on, more and more bits of the original ship were replaced as they decayed. This raised a question among ancient philosophers which has come to be known as the ‘Theseus paradox.’  If a stage was reached when all the pieces of the original ship were thus replaced was the current ship still the Ship of Theseus? If not, at what point did the original ship cease to exist? 
The discussion about this issue was first noted by the Greek writer Plutarch in his Life of Theseus. Plutarch writes: ‘The ship on which Theseus sailed with the youths and returned in safety, the thirty-oared galley, was preserved by the Athenians down to the time of Demetrius Phalereus. They took away the old timbers from time to time, and put new and sound ones in their places, so that the vessel became a standing illustration for the philosophers in the mooted question of growth, some declaring that it remained the same, others that it was not the same vessel.’ 
Philosophers today still discuss the Theseus paradox, namely, how much can something change and still retain its identity? Philosophers differ on this point, but the answer seems to be that identity can be said to persist in the face of change if there is some essential form of continuity. 
Thus, in the case of Theseus’ ship, the fact of its having a continuous history even while a growing number of its parts were replaced means that it could be said to be the same ship. In a similar way a human being can be said to be the same person although billions of cells in their bodies are replaced every day. 
The issue of continuity and change is also raised by the study of Christian doctrine. This is because Christians have historically held two convictions concerning Christian doctrine. The first is that the content of Christian doctrine is unchanging. The second is that Christian doctrine rightly develops. 
The classic statement of these two convictions can be found in a book called the Commonitory which was written by Vincent of Lerins in the fifth century. In this book Vincent addresses the question as to how we determine what is Catholic orthodoxy and what is ‘heretical depravity’. The famous answer he gives to this question has come to be known as the ‘Vincentian canon’ (canon meaning ‘rule’).
It states: ‘… all possible care must be taken, that we hold that faith which has been believed everywhere, always, by all. For that is truly and in the strictest sense ‘Catholic,’ which, as the name itself and the reason of the thing declare, comprehends all universally. This rule we shall observe if we follow universality, antiquity, consent. We shall follow universality if we confess that one faith to be true, which the whole Church throughout the world confesses; antiquity, if we in no wise depart from those interpretations which it is manifest were notoriously held by our holy ancestors and fathers; consent, in like manner, if in antiquity itself we adhere to the consentient definitions and determinations of all, or at the least of almost all priests and doctors.’ 
The point that Vincent is making is not that Christian tradition replaces Scripture as the source of orthodox Christian teaching. It is rather that when deciding how to interpret Scripture rightly we need to attend to what CS Lewis later called ‘mere Christianity,’ the basic pattern of Christian teaching that has existed in Christian churches around the world since apostolic times and which is summarised in the Catholic Creeds. 
Noting Paul’s words in Galatians 1:8: ‘Though we, or an angel from heaven, preach any other Gospel unto you than that which we have preached unto you, let him be accursed.’   
Vincent further adds: ‘To preach any doctrine therefore to Catholic Christians other than what they have received never was lawful, never is lawful, never will be lawful: and to anathematize those who preach anything other than what has once been received, always was a duty, always is a duty, always will be a duty. Which being the case, is there any one either so audacious as to preach any other doctrine than that which the Church preaches, or so inconstant as to receive any other doctrine than that which he has received from the Church?’ 
On the basis of  quotations like the two just given, Vincent is often viewed as an arch conservative who believes that Christian doctrine must remain entirely static. However, to view him in this light is to ignore the fact that he goes on in the Commonitory to respond to the question ‘Shall, there, then, be no progress in Christ’s Church?’ with the answer that there is ‘all possible progress.’ 
However, this has to be ‘real progress, not alteration of the faith’. 
‘For progress requires that the subject be enlarged in itself, alteration, that it be transformed into something else. The intelligence, then, the knowledge, the wisdom, as well of individuals as of all, as well of one man as of the whole Church, ought, in the course of ages and centuries, to increase and make much and vigorous progress; but yet only in its own kind; that is to say, in the same doctrine, in the same sense, and in the same meaning.’ 
The issue that Vincent’s work leaves us with is how there can be doctrinal progress (and therefore necessarily some kind of change) without any doctrinal alteration. 
Over the course of Christian history this issue has been much debated by Christian theologians, but what seems to me to be the most persuasive account of how there can be change without alteration is provided by the nineteenth century Church of England writer Jacob Mozely in his book The Theory of Development. Mozley argues that the kind of doctrinal progress advocated by Vincent should be seen in terms of what he calls ‘explanatory  progress’. 
He writes: ‘Development is explanation; explanation is development. A man in conversation makes an assertion, which another misapprehends; in reply, he explains the meaning, or develops the meaning of his assertion. His meaning is exactly the same with what it was before; it is in order to show what it was before that the explanation is given; the meaning before the explanation or development of it, and the meaning after, are by the very nature and aim of the process the same.’
He continues, ‘The case is not infrequent of a single arguer having to maintain in conversation a particular point against a whole circle of opponents. He adheres firmly, consistently, and with all unity and simplicity, to the one point which he defends, and is only bent on defending it. But, with that one object in view, what a vast formation of language does he raise as he goes on! What distinctions accumulate, and what protests and safeguards grow up out of, and surround the original statement! He would be surprised at the end of the argument to see the edifice he had built. And yet nobody would say that his idea had altered, and was not just the same as it was when he began.’
This explanatory understanding of the development of doctrine helps us make sense of Vincent’s claim that doctrine can be ‘novel but not new’ (noviter non nova). If the development of doctrine means explaining the old truths of the Christian revelation in new ways, then it makes perfect sense to say with Vincent that to develop doctrine means to ‘designate by new and appropriate words some article of faith, which is, of itself, traditional’. 
To explain something means precisely to designate something by the use of ‘new and appropriate words’. That is what explanation is. Explanation does not involve novelty in the sense of changing an accepted truth. Rather, it involves clarifying through the use of new language what that truth is. This, for example, is what the Fathers did at the Council of Nicaea when they declared for the first time that the Son is ‘consubstantial’ (homoousios) with the Father as a way of safeguarding the truth set down by John that ‘the Word was God’ (John 1:1). 
This understanding of the development of doctrine also helps us to make sense of the current debate in the Church of England about whether the Church can introduce some form of recognition of same-sex marriage without changing its doctrine (something that General Synod has ruled out). 
The suggestion made by supporters of same-sex marriage is that it can be seen as a legitimate form of doctrinal development. The Church’s Faith and Order Commission is due to report on this issue in the autumn, but it seems clear to me that if we follow Mozley in seeing doctrinal development as ‘explanatory progress’, recognising same-sex marriage cannot be seen as a legitimate development. 
As Darrin Belousek writes in his book Marriage, Scripture and the Church, ‘Scripture, consistently, presents a single picture of marriage and approves a single pattern of sexual relations: male-female union. Jesus summarizes this witness: ‘the two’ of ‘male and female’ joined into ‘one flesh.’ The Holy Spirit has woven this pattern of holy union throughout Scripture, from Genesis to Revelation, in the form, function, and figure of marriage.
‘Tradition, East and West, also has consistently taught a single standard of sex and marriage: marriage is man-woman monogamy; all sex outside man-woman monogamy is sin. This doctrine has been taught always by the church, beginning with the apostles’ testimony to Jesus teaching; it has been proclaimed throughout the worldwide church, among all people in every place and epoch, as God’s will for sex and marriage; it has been articulated by apologetic writings and theological treatises, transmitted through baptismal catechesis and canonical discipline, celebrated in monastic vows and nuptial rites.’
The Church of England’s view of marriage, authoritatively outlined in the marriage service in the Book of Common Prayer and in Canon B.30, reflects exactly the same Scriptural and Catholic teaching summarised by Belousek. To recognise same-sex marriage would not be to explain this teaching but to depart from it. In Vincent’s language it would not be doctrinal progress but ‘heretical depravity’.