Cassiodorus, who lived from 485-585AD, was well educated and skilled in Greek, was a Roman senator (called "Flavius Magnus Aurelius Cassiodorus") at one point, was Praetorian Prefect for Italy under the Ostrogothic kingdom, established a library for Christians, and had professional Latin translators that translated from older Greek manuscripts into Old Latin. Eventually, Cassiodorus traveled from Italy to Constantinople, collected Greek and Old Latin manuscripts, translated manuscripts and literature from Greek into Latin, proofread the manuscripts with his team of translators to ensure there were no errors, studied and read both the Scripture and old classical literature and Scriptures, and founded a library/school/study center named Vivarium. Cassiodorus is a credible witness to 1 John 5:7-8 as he went to and had Greek and Latin manuscripts that contained 1 John 5:7 in order to literally quote 1 John 5:7-8 in his Complexiones.
For more information, see The Comma Calmly Considered - The Complexiones (circa 575) of Cassiodorus by Mike Ferrando:
https://www.academia.edu/96854779/19_The_Comma_Calmly_Considered_The_Complexiones_circa_575_of_Cassiodorus