Elsewhere, I’ve written about some limits of patristic evidence, but in this post, I want to give a positive example of how it can be used.
At James 5:4, the vast majority of manuscripts have ἀπεστερημένος (defraud, deprive of), but the earliest extant manuscripts (Codices Sinaiticus and Vaticanus) have ἀφυστερημένος (withhold). In context, there is not much difference, and the two readings differ only with regard to two letters, -πε- or -φυ-. Still, there are only three Greek New Testament manuscripts extant for this part of James before the 9th century, and they are divided. Codex Alexandrinus supports ἀπεστερημένος, and this is the reading adopted by the Tyndale House Greek New Testament.
As it happens, Didymus the Blind cites James 5:4 in his Commentary on Genesis, and in his citation, he has ἀπεστερημένος (with the majority of manuscripts). I came across the quote when looking at Mike Arcieri’s McGill PhD dissertation on Didymus. That would be early support for ἀπεστερημένος, roughly contemporary with the earliest Greek manuscripts of James, but even if this was not Didymus’ text, this particular citation is still valuable.
When I checked the SC edition of Didymus, I noticed that it was edited from one of the Tura Papyri (Brent Nongbri has some helpful information on the Tura Papyri here). Specifically, Codex IV of the Tura Papyri contains Didymus’ Commentary on Genesis, and it dates to the 6th or 7th century.
The significance here is that regardless of what Didymus’ text of James 5:4 actually was (that is, even if we take a hyper-skeptical position that the Tura Papyrus does not give us Didymus’ text of James 5:4), we still have a witness to ἀπεστερημένος in James 5:4 from the 6th or 7th century, because that is the age of the manuscript of Didymus’ Commentary on Genesis that has the citation of James 5:4. This witness pre-dates the vast majority of Greek manuscripts of James from the 9th century and later. Admittedly, not by much, and admittedly, Codex Alexandrinus is still the earliest support for ἀπεστερημένος, but we no longer have a gap between the 5th and 9th centuries in which there are no witnesses to James 5:4.
At James 5:4, the vast majority of manuscripts have ἀπεστερημένος (defraud, deprive of), but the earliest extant manuscripts (Codices Sinaiticus and Vaticanus) have ἀφυστερημένος (withhold). In context, there is not much difference, and the two readings differ only with regard to two letters, -πε- or -φυ-. Still, there are only three Greek New Testament manuscripts extant for this part of James before the 9th century, and they are divided. Codex Alexandrinus supports ἀπεστερημένος, and this is the reading adopted by the Tyndale House Greek New Testament.

When I checked the SC edition of Didymus, I noticed that it was edited from one of the Tura Papyri (Brent Nongbri has some helpful information on the Tura Papyri here). Specifically, Codex IV of the Tura Papyri contains Didymus’ Commentary on Genesis, and it dates to the 6th or 7th century.
The significance here is that regardless of what Didymus’ text of James 5:4 actually was (that is, even if we take a hyper-skeptical position that the Tura Papyrus does not give us Didymus’ text of James 5:4), we still have a witness to ἀπεστερημένος in James 5:4 from the 6th or 7th century, because that is the age of the manuscript of Didymus’ Commentary on Genesis that has the citation of James 5:4. This witness pre-dates the vast majority of Greek manuscripts of James from the 9th century and later. Admittedly, not by much, and admittedly, Codex Alexandrinus is still the earliest support for ἀπεστερημένος, but we no longer have a gap between the 5th and 9th centuries in which there are no witnesses to James 5:4.