Modern Christians, in our weakness and dispersion, need to be more careful than ever about heresy: and this means steering a path between the false negative of failing to reject 'liberal' and worldly heresies; and the false positive of falsely detecting heresy and rejecting real Christians of other denominations.
Consider some of the Christian authorities I have been reading with intensity over the past year or two, in no particular order. By 'authorities' I mean that I personally regard them as authorities:
Fr Seraphim Rose
St John Maximovitch
Fr Herbert Kelly
Protopresbyter Michael Pomazansky
The author of The Way of a Pilgrim
Plus of course The Book of Common Prayer (Thomas Cranmer, Miles Coverdale) and The Bible specifically in the Authorized Version (William Tyndale, Miles Coverdale, Lancelot Andrewes etc).
The fact is that some of these Authorities would have regarded some of the others as heretics; and would have wanted nothing to do with them, demanded their conversion, or their suppression.
Fr Seraphim Rose - Russian Orthodox
C.S Lewis - Anglican, moving from Low to High Church
JRR Tolkien - Roman Catholic, very traditional
Blaise Pascal - Roman Catholic, but Jansenist = explicitly 'heretical'
St John Maximovitch - Russian Orthodox
Archbishop Averky - Russian Orthodox
Fr Herbert Kelly - Anglo-Catholic monk
Charles Williams - Anglican, mainly Anglo Catholic
Protopresbyter Michael Pomazansky - Russian Orthodox
Thomas Traherne - Anglican mainstream of 17th century
Peter Kreeft - Roman Catholic, pre Vatican II tending
The author of The Way of a Pilgrim - Russian Orthodox
But I align with C.S Lewis in this matter.
There are real Christian heresies; but there have also been intense inter-denominational disagreements and conflicts within Christianity about matters that we now perceive not to be concerned with heresies.
Sometimes these differences may damage the probability of salvation (for an individual person, or for an average group), or the scope of theosis (sanctification) - but they are not heresies.
So, I would personally regard the Reformed churches rejection of monasticism as diminishing the scope of theosis, and I would regard the rejection of veneration of the Blessed Virgin Mary as making Christianity harder for the average Christian - but neither of these would amount to heresies.
I would regard some recent Roman Catholic dogmas such as the Immaculate Conception and Papal Infallibility as excessively legalistic attempts at doctrinal precision and political control; but I would not regard them as heresies.
One of the very few places in his entire ouvre where Lewis displays principled anger is in a letter when he begs Lewis's friend the (then) Benedictine monk Dom Bede Griffiths to please stop trying convert him to Roman Catholicism - and outlaws that topic of conversation.
For Lewis, this behaviour from Griffiths was implicitly based on the assumption that Anglicans were outwith the legitimate bounds of Christianity; hence a denial of 'Mere Christianity' and an act of denominational exclusivism.
Although he began as mainly a Low Church Protestant Anglican, and ended-up much more of an Anglo Catholic, Lewis personally seems never to have considered converting to a different Christian denomination (of course he might well have changed his mind later, if he had lived another 25 years - e.g Walter Hooper has argued cogently that Lewis would probably have become a Roman Catholic).
I think Lewis regarded deliberate efforts in this direction of inter-denominational 'poaching' as not just misguided but likely to be dangerous and damaging to Christian unity and therefore to the mystical Church.
(Of course, behind this attitude of Lewis's lies his belief that there is indeed a real and specifically Christian unity across denominations; which is something atht not all of the above authorities would have agreed upon.)
On the other hand, Lewis seems not to have objected to people changing denominations, so long as this was:
1. Done for properly Christian reasons, and
2. Did not lead to denigration of the previous denomination.
Yet at the level of dogmas, doctrines, liturgies, beliefs, devotional practices, use of Scripture, structure of authority and innumerable other denominational definitions - there is ample reason for mutual accusations of heresy among my authorities listed above; and for each of them ample reasons for exclusivism.
While, on the other hand, the definition of Mere Christianity which unites all these authorities is so simple and basic as to be insufficient as a basis for a Christian life.
It seems, therefore, from the Mere Christianity perspective I share with Lewis:
1. That we must have Christian denominations (because the Mere Christianity that is shared between all denominations is insufficient for a Christian life)
2. Yet none of the Christian denominations are exclusively correct.
3. So that each denomination has advantages and disadvantages for particular persons and on average, and in particular circumstances. For a specific person, group, time or place; the choice of denomination may be critical to their Christian life. So denomination does matter - but not in a fashion related to heresy.
4. Mere Christians should strive neither to encourage nor discourage, but instead to play-down the significance of inter-denominational conversions.
5. Once a denomination is Christian, comes within Mere Christianity, we should try to avoid discussions - and especially debates - about heresy. This line of argument should be reserved for those few core matters that must be shared by all Christians.
6. Denominations should be free to manage their internal affairs, and be strong in their distinctive perspectives and emphasis; but without recourse to building internal denominational unity through hatred of or differentiation from other Christian denominations. Denominational unity should be defined against 'the world', and against non-Christian religions; but not against other Christian denominations.
7. There should be no attempt to fuse denominations, since this leads to dilution and weakness and provokes resentment and schism; nor to make formal organizational agreements and declarations - these tend to be bureaucratic and to encourage careerist thinking about Christianity; remembering true Christian unity is mystical, not institutional.
8. Thus the relationship between denominations should therefore resemble the relationship between autonomous nations allied in a war; and the warfare metaphor clarifies why Christian denominations must be allies, and must not engage in mutual denunciation, since they are confronted by an enemy who would destroy them all.
Once it has been determined on which side a denomination is fighting in the great spiritual war - and in that war there are only two sides - then all other considerations should be subordinated to that crucial fact.