Here is a good discussion of the apologetic claim that the 3rd-century “church father” Cyprian quoted the “Johannine Comma”. Some Christians use Cyprian’s quote as “proof” that the corrupted reading of 1 John 5:7 is authentic, but the evidence shows that this is false. Below are some relevant quotes from the article. The author gives multiple reasons for why Cyprian was not quoting the version of 1 John 5:7 that Christians want him to quote. The full article may be accessed here: https://www.academia.edu/29542906/Did_Cyprian_Quote_the_Comma_Johanneum?auto=download
Hat-tip to brother Shaad for the reference. Jazak Allah Khair.
“First, it is not a verbatim quotation. A verbatim quotation would reference the “Father, Word, and Holy Spirit,” a distinctive phrase that occurs nowhere else in Scripture. Cyprian quotes “Son,” (filio) not “Word” (verbum). Although verbatim quoting is not always determinative, it plays an important role in evaluating patristic citations. In the immediate context of the quotation (et tres unum sunt), Cyprian references many Scriptures, including Gen 7:20; Matt 12:20; John 10:30; 2 Cor 11:2; Eph 5:23; and1 Pet 3:20. It is therefore possible that he was referencing the language of Matt 28:19 combined with 1 John 5:8. The quotation does not reference anything distinctly found in the Comma.”
“A fifth problem concerns why Augustine, who lived at the time of the Arian controversy, did not bother to invoke the Comma in his writings. […]  Cyprian’s writings were revered for over four centuries as one step below Scripture, and Augustine so revered Cyprian that he presented at least a dozen sermons celebrating a memorial feast to Cyprian. Nowhere in any of his writings does Augustine quote the Comma. Such a scenario is unlikely if Cyprian quoted the Comma.
“Cyprian finds references to Christ as “the Word of God” in Psalms, Proverbs, Isaiah, the Gospel of John, and Revelation but never mentions the Comma [in Epistle XXIV.2], the most explicit testimony to Christ as “the Word” outside of John’s gospel. While many other instances could be considered debatable, this lack of quotation strongly suggests that Cyprian never saw the Comma. Given his chain reference method of citing every instance of Christ as the Word in this treatise, his failure to cite the Comma is best explained by the lack of the phrase in his text(s).
“Because of the failure of the grammatical argument, the overwhelming external evidence, and the lack of citation by the Greek church fathers, it is safe to conclude the Comma is a Latin interpolation. Therefore, even if Cyprian did quote the Comma then that merely moves the date of the first known quotation back a century. It does not change the fundamental reality: not one scrap of Greek evidence of the Comma exists in the first ten centuries, and all of the extant evidence comes from one secondary language, Latin. More problematic is that despite the dispute over the Trinity that covered several centuries, the citations (both real and alleged) of the Comma are small in both number and geographical distribution.“