Exodus 23:19 - Joseph Benson’s Commentary on the Old and New Testaments

Thou shalt not seethe a kid in his mother's milk It is remarkable that this command, extraordinary as it is, is repeated Exodus 34:26, and Deuteronomy 14:21, and that, as here, in connection with the offering of the first-fruits. Hence it has been conjectured that it has a reference to the payment of these fruits, and to some superstitious practices which the Pagans used on these occasions, who were wont, it seems, when they had gathered in all the fruits of the earth, to boil a kid in its mother's milk, and “to sprinkle the trees, and fields, and gardens, with the broth in a magical manner, to make them more fruitful the following year.” See Dr. Cudworth, On the Lord's Supper, page 14. Some, however, with an appearance of probability, take this for a prohibition against offering any animal in sacrifice when it was milky and unformed, or before it was eight days old, till which time it was to be left with its dam, Exodus 22:30. And others, again, consider the precept as being chiefly intended, like many other of God's laws, to prevent cruelty toward the creatures, and to inculcate a mild and tender disposition.