(Part 17 can be read here)
(This part: More about the church fathers and 1 Jn.5:7, specifically the Latin church fathers and the dogma of the Trinity/1 Jn.5:7)
And in Facundus, an ecclesiastical scribe in the sixth century, one can read, precisely with regard to the above quote from Cyprian:
"Of the father, the son, and the holy spirit, the apostle John says: »three bear witness, the spirit, the water, and the blood, and the three are one», signifying with the spirit the father... This testimony by the apostle John has in the blessed Cyprian, in his book or letter about the unity, been understood to be about the father, the son, and the holy spirit.”
Tischendorf states that even in ancient Greek codices one finds that remark written in the margin next to 1 Jn. 5: 7 - 8, that »the spirit, the water and the blood signify the father, the son, and the holy spirit».
Besides that the above quote from Augustine and Facundus shed the right light on the quote in Cyprian, they are also, in and of themselves, strong evidence against the spurious words in 1 Jn.5: 7 - 8.
The supposition, that the spirit, the water, and the blood signify the father, the son and the holy spirit, were quite close at hand for the expositors of the Bible at a time, which made a limitless use of the archetypical, symbolic and allegorical interpretation.That the teats of the bride in the Song of Songs signified the old and new testament, the faces in the cherub chariots the four evangelists, David Christ, Bathsheba the church and Uria the devil - these and such sometimes unsavory offsprings of a fantastic Bible interpretation, which one still today can come across, count to some extent their ancestry from the church fathers. Others have their origins in the monk cells of the Middle Ages.
The originator of the spurious verse and the one who first quotes it as truly written by the apostle John, is the in history most infamous African bishop Vigilius (toward the end of the fifth century). In a work directed against the Arian Varimad, which he published under the assumed name Idacius Clarus, the following namely appears:
»John the evangelist says to the Parthians*: there are three that bear witness on earth: the water, the blood and the flesh, and the three are in us; and there are three that bear witness in heaven: the father, the word, and the spirit, and these three are one.(1)
* Vigilius had read in Augustine that John's first letter should have been written to the "Parthians". This outlandish information, which is found in Augustine's treatise »Evangelical questions», is based on a misunderstanding. In the Greek church father Clemens Alexandrinus appears namely a note that one of John's letters is written to "virgins". The word virgin is parthenos in Greek. This word Augustine, who himself admits his ignorance of Greek, might have thought means the folk name Parthian.
(1) In the original language: »Johannes evangelista ad Parthos Tres sunt, inquit, qui testimonium perhibent in terra, aqua sanguis et caro et tres in nobis sant;et tres sunt, qui testimonium perhibent in coelo, pater, verbum et spiritus, et hi tresunum sunt.»