Unfortunately, Hieronymus Jerome's commentary on Galatians has only recently been translated into English, and there is no preview in Amazon or Google Books. I am trying to determine exactly where in this commentary this statement is made (in Latin, of course).Hieronymus VII,4:22 (Zahn, Geschichte, p.499 “From this place all the way up to where it is written “they which are of the faith, are blessed with faithful Abraham” (v.9), Marcion erased from his apostle...”.
The Latin text of Commentariorum In Epistolam Beati Pauli Ad Galatas is supposed to be in Migne's Patrologia Latinae vol 26, section 377. So, not yet having tried to look it up, I ask "what the hell does 'VII,4:22' mean?" Does this commentary have at least seven books? All the egg-heads cavalierly speak about it as if even the wash ladies in Latvia knows where to find this passage. To be honest, I am stumped, and vol 26 will not download from archive.org
FWIW, Jerome is supposedly following a lost commentary of Origen here.
DCH