John Calvin Commentary 2 Corinthians 11:24

"Of the Jews five times received I forty [stripes] save one." — 2 Corinthians 11:24 (ASV)

From the Jews. It is certain that the Jews had at that time been deprived of jurisdiction, but as this was a kind of moderate punishment (as they termed it), it is probable that it was allowed them. Now the law of God was to this effect: that those who did not deserve capital punishment should be beaten in the presence of a judge (Deuteronomy 25:2, 3), provided not more than forty stripes were inflicted, lest the body should be disfigured or mutilated by cruelty.

Now it is probable that, in process of time, it became customary to stop at the thirty-ninth lash, lest perhaps they should on any occasion, from undue warmth, exceed the number prescribed by God. Many such precautions, prescribed by the Rabbis, are to be found among the Jews, which make some restriction on the permission that the Lord had given.

Hence, perhaps, in process of time (as things generally deteriorate), they came to think that all criminals should be beaten with stripes to that number, even though the Lord did not prescribe how far severity should go, but only where it was to stop. Alternatively, perhaps you prefer to accept what others state: that they exercised greater cruelty on Paul.

This is not at all improbable, for if they had been ordinarily accustomed to practice this severity on all, he might have said that he was beaten according to custom. Hence, the statement of the number is expressive of extreme severity.