Analysis of information sources in references of the Wikipedia article "رسالة يوحنا الأولى" in Arabic language version.
There are several ways of accounting for that omission and among others, it may be said, 1st, that this omission happened by the neglect of some ignorant copyists, who, after having written the first words of the 7th verse 'there are three, that give testimony', by a mistake of the eyes, skipped over the remaining part of the text, and passed on to the immediately following text, where the same words recur; for such mistakes often take place in transcribing, especially when the two verses and the two periods begin and end with the same words. Another reason of this omission is given by the author of the prologue to the seven Catholic epistles... (Vulgate Prologue section translation)... By these words he not obscurely alludes to the Marcionites or Arians, who designedly erased this verse from all the copies they could get into their hands; for they well understood that by that one testimony their cause was undone. With a like perfidy, St. Ambrose, (lib. iii de spiritu sancto cap. 10.) reproaches the Arians, who had expunged these words from the Scriptures: Because God is a Spirit, 'Which passage, says the holy doctor addressing the Arians, you so well know to be understood of the Holy Ghost, that you have erased it from the copies of your scriptures, and would to God! you had only expunged it from yours and not also from those of the church.
There are several ways of accounting for that omission and among others, it may be said, 1st, that this omission happened by the neglect of some ignorant copyists, who, after having written the first words of the 7th verse 'there are three, that give testimony', by a mistake of the eyes, skipped over the remaining part of the text, and passed on to the immediately following text, where the same words recur; for such mistakes often take place in transcribing, especially when the two verses and the two periods begin and end with the same words. Another reason of this omission is given by the author of the prologue to the seven Catholic epistles... (Vulgate Prologue section translation)... By these words he not obscurely alludes to the Marcionites or Arians, who designedly erased this verse from all the copies they could get into their hands; for they well understood that by that one testimony their cause was undone. With a like perfidy, St. Ambrose, (lib. iii de spiritu sancto cap. 10.) reproaches the Arians, who had expunged these words from the Scriptures: Because God is a Spirit, 'Which passage, says the holy doctor addressing the Arians, you so well know to be understood of the Holy Ghost, that you have erased it from the copies of your scriptures, and would to God! you had only expunged it from yours and not also from those of the church.