His argument was that under Emperor Constantine, the gospel was so significantly altered as to render our modern manuscripts inaccurate and unreliable.
This meant that the Roman Empire would have an effective tool for subduing its subjects.
My brother accepts the existence of Jesus as an historical figure, but believes that history was rewritten by the Romans to suit their interests, such as was done under Stalin.
Though I do not believe this claim, I am interested in knowing how to defend Christianity on this issue.
Your brother is asking a thoughtful and good question...
The answer is rather simple, but can be very instructive in illustrating the methodological problems with a number of such "conspiracy theories" of some power structure (e.g. the Roman empire, the Church, a specific power structure, some wealthy Roman family, etc.) radically altering the NT to suit their purposes.
Let's examine first the issue of WHAT DATA would be required to 'prove' your brother's thesis.
Logically, your brother would have to produce evidence of:
Now, the odd thing about this, is that if we have #1 (some unaltered original) from which to determine that an alteration occurred, THEN WE HAVE an 'accurate and reliable' manuscript! In other words, to prove his thesis is to refute it! If the empire had done alterations (without disposing of all the originals) then we would have the originals with which to base our rejection of the Empire's fabrications!
What this implies is that IF WE HAVE manuscripts that can be dated to PRE-CONSTANTINE years (i.e. in the 1st, 2nd, or 3rd centuries), then we, BY DEFINITION, have manuscripts that are 'unaltered' by CONSTANTINE--and hence your brothers supposition becomes trivial. (Not altogether 'untrue', for we DO FIND alterations in later texts--but these changes can be 'weeded out' on the basis of the earlier, 'unaltered ones'.)
Interestingly enough this is EXACTLY the case. Most of our NT texts are based on existing manuscripts that were in existence LONG BEFORE the Constantine deal!
Consider the 'hard data' of textual criticism, archeology, and paleography:
Below are the major EARLY manuscripts in existence today, with dates and general contents. (All of the below data are from ATNT, MTNT3,EMMT, COMFORT)
First, let me point out that these texts are NOT simply 'tiny fragments'. Let's look at each of the two collections.
The Beatty papyri.
The major papyri in this collection are p45, p46, p47.
The Bodmer papyri
The major papyri in this collection are p66, p72, p75.
But what about OTHER papyri, not included in the Beatty and Bodmer collections? Are there any other mss. that would give us clues as to how early the NT was written and in circulation?
Aland and Aland (ATNT: 85f) summarize the significance of p52 and the Beatty collection:
We cannot conclude this survey of the papyri without some further comments on the truly amazing discoveries of the past generation. The critical significance of p52, which preserves only a fragment of John 18, lies in the date of 'about 125' assigned to it by the leading papyrologists. Although 'about 125' allows for leeway of about twenty-five years on either side, the consensus has come in recent years to regard 125 as representing the later limit, so that p52 must have been copied very soon after the Gospel of John was itself written in the early 90's A.D. (with the recent discovery of p90 another second century fragment of the Gospel of John is now known). It provides a critical witness to the quality of the New Testament textual tradition, further confirming it by exhibiting a 'normal text', i.e., attesting the text of today (that of Nestle-Aland26 and GNT3). While it is true that papyri from the third century were known before the discovery of the Chester Beatty papyri, none of them was as early as p46, which contains the Pauline letters and has been dated 'about 200' (with some leeway on either side). But more significantly, all the early papyri known previously contain no more than a few verses of the New Testament text, with the exception of p15 from the third century which preserves almost a whole leaf. Now for the first time entire New Testament writings became available from the early period.
Although that's enough data to refute your brother's position, let's go a step farther...
Now let's as the question as to how much ACTUAL modification of the texts occurred during the 4th century by the Empire or Church. Some will say that wholesale modifications of the text occurred during this period. These 'hard data' mss. above SHOULD exert control over these theories of massive recensions and revisions. And the significance of textual finds in this direction is argued by Aland (ATNT: 87):
The implications of Papyrus Bodmer XIV/XV (p75) of the gospels of Luke and John went even further. Written somewhat later (than p66), at the beginning of the third century, it comprised twenty-seven almost perfectly preserved sheets together with a part of their binding. This papyrus marked another revolution in our understanding of how the New Testament text developed: its text proved to be so close to that of Codex Vaticanus (B) that the theory of recensions, i.e., of thoroughgoing revisions of the New Testament text made in the fourth century, was no longer defensible. One of the main pillars supporting the dominant theory of New Testament textual history was now demolished.
It should be rather clear from the above that the 'hard data' of textual criticism (NOT a 'theological thing'!) documents quite strongly that we have (1) early, 'unaltered' documents from the pre-Constantine years; and (2) that even the manuscripts DURING/AFTER Constantine do not show significant alteration from the pre-Constantine ones...
So, even if the Roman empire had a 'motive', they obviously did not act upon it...
Thus the NT manuscripts we have today can be trusted to reflect the original pre-Constantine period.
Hope this helps...