"Let him who is without sin cast the first stone" is straightforward Scripture

The passage (John 7:53-8:11) is absent from the earliest and best Greek manuscripts of John, including papyri and codices that scholars consider the most reliable witnesses to the original text. Where later manuscripts do include the story, they don't agree on where it belongs: some place it after John 7:36, others after John 21:25 (the Gospel's actual ending), and a handful of manuscripts insert it into Luke instead, after 21:38 or 24:53. That kind of floating placement, moving between two different Gospels across different manuscript traditions, is the textual-criticism signature of a real, cherished story being inserted by scribes who wanted to preserve it, not the signature of an original composition sitting in a fixed spot.

Elijah Hixson and Peter Gurry's Myths and Mistakes in New Testament Textual Criticism adds a second, independent line of evidence beyond the manuscripts themselves: the early Greek commentators. Several ancient scribal marginal notes on manuscripts that do contain the passage flag the problem directly: one from the 1100s reads, "this does not appear in the majority of copies; nor is it commented upon by the divine fathers whose interpretations have been preserved, specifically, by John Chrysostom and Cyril of Alexandria." That matters because Chrysostom and Cyril both wrote extensive verse-by-verse homilies working through the whole Gospel of John in the 4th-5th centuries. If the passage had been in their copies, a scene this dramatic would be an obvious thing to preach on, and neither one does.

Kenneth Bailey's chapter on the passage in Jesus Through Middle Eastern Eyes takes the story seriously as an authentic Jesus tradition even while granting the manuscript problem: he reads it as an agrapha, a genuine unwritten memory of Jesus passed down orally in the early church before finally being recorded and, eventually, inserted into copies of John. Most modern translations print the passage with a bracket, italics, or a footnote flagging exactly this: the story is old and almost certainly reflects a real event, but its place inside John's Gospel specifically is a manuscript question serious scholarship doesn't paper over.