People are demanding evidence when
in the final analysis the veracity of 1 John 5:7 is a matter of faith although evidence indeed plays a part in faith.
Hebrews 11:1 Now faith is the substance of things hoped for, the evidence of things not seen.
In addition, the demand of evidence is like a person going to a restaurant and ordering a dinner. Someone else does all the preparation and work and you enjoy the meal.
Not that that is wrong in and of itself, but my practice (when I decide to do it) is to give a high level citation and a sample or two of quotations then if an individual is serious about this (or any other matter) they will pursue it.
Over the years I have studied the matter of the Johannine Comma (aka 1 John 5:7). and have made a faith decision that it is part of the word of God.
Now-a-days you can very easily take another research short cut : go to the web and search for "johannine comma".
You will find a multitude of references both pro and con, enough to keep you going for several years. Myself, I like to purchase books as well and actually had no choice before the modern advent of the web (yes, I am that ancient).
Here is a good book (everything is documented).
Before I cite the book
in my experience, most people who are critical of the author have not read his book(s). Why bother then with a debate?
Then there is the sanctimonious statement (and I point this out because I have judged myself guilty) to end the debate with something like "I believe the bible" and usually (but not always) means "I don't want to do the work".
In fact is this not the practice of many of us?
Again like the man who sees his face in the mirror of the word of God will go his way and forget what he saw (I'm talking about my own sanctimonial judgements). Thankfully He is diligent to remind us of our pride.
James 1
23 For if any be a hearer of the word, and not a doer, he is like unto a man beholding his natural face in a glass:
24 For he beholdeth himself, and goeth his way, and straightway forgetteth what manner of man he was.
25 But whoso looketh into the perfect law of liberty, and continueth therein, he being not a forgetful hearer, but a doer of the work, this man shall be blessed in his deed.
26 If any man among you seem to be religious, and bridleth not his tongue, but deceiveth his own heart, this man's religion is vain.
Many publications (in full and in part) are on the web with URL's of private libraries you may have to pay for - what is it worth to you to study (be diligent) to show thyself approved?
Here is the citation:
A History of The Debate Over 1 John 5:7-8, Michael Maynard, Comma Publications Tempe, AZ, 1995.
On page 37 he cites the quotation of Cyprian (AD250) in his work:
De catholicae ecclesiae unitate
The Lord says I and the Father are one and like wise it is written of the father and the Son and the Holy Spirit and these three are one
As well as other references to the Comma by this Church Father.
Later he (Maynard) also goes into the historic fact that during the church-wide controversy concerning the doctrine of the Holy Trinity (AD100-400) Arian (non-trinitarian) believers would break into book repositories and burn manuscripts and in particular 1 John. That is a possible reason that there are so few 1 John manuscripts extant.
Maynard's book contains more rererences to other writings and citations than the text of the book itself with over an additional 100 pages of reference indicies.
It's a good starting place for those who are interested in facts (as well as the challenge of a worthy debate).
HankD