Erasmus claimed in the preface to have consulted the oldest and best manuscripts... (Hotchkiss, Valerie and Price, David, The Reformation of the Bible & The Bible of the Reformation, New Haven: Yale University Press, 1996, p. 100). He said his text was “solidly based” (de Jonge, Henk J., $Novum Testamentum a Nobis Versum: The Essence of Erasmus Edition of the New Testament,# Journal of Theological Studies 35, October 1984, p. 400). Erasmus suggested that he had consulted many manuscripts (Cambridge History of the Bible, vol. III, p. 60).
Erasmus further verified his Greek New Testament with scripture quotations seen in the writings of early Christian writers. His Greek text is so perfect, because he spent the first fifteen years of his studies almost wholly given to translating the early Christian writers of the first few centuries after Christ. In these writings from the 2nd, 3rd, and 4th centuries, one finds evidence for the Bible's oldest readings. They usually predate, by several hundred years, the Vaticanus and Sinaiticus MSS, from which modern translations get their readings. Froben published Erasmus' work on the ‘Fathers,’ as a series which included, Cyprian, Irenaeus, Chrysostom, Basil, Ambrose and numerous others. Being a theologian, Erasmus knew the origin of heretical omissions. “[I]n many places the virus still lurked of...Marcion,” he noted (Bainton, p. 264).

https://textusreceptusbibles.com/Editorial/Erasmus