Verses 9, 10. - And Onan knew that the seed should not be his; and it came to pass, when - literally, and it was if, i.e. whenever (cf. Ewald, Hebrews Synt., § 3456) - he went in unto his brother's wife, that he spilled it on the ground (literally, destroyed to the ground), lest that he should (or, so as not to) give seed to his brother. And the thing which he did displeased (literally, was evil in the eyes of) the Lord: - the word Jehovah is employed not because the writer was a late interpolator, but because the sin of Onan was an offence against the sanctity and prosperity of the theocratic family (Hengstenberg) - wherefore he (i.e. Jehovah) slew him also (vide supra).

38:1-30 The profligate conduct of Judah and his family. - This chapter gives an account of Judah and his family, and such an account it is, that it seems a wonder that of all Jacob's sons, our Lord should spring out of Judah, Heb 7:14. But God will show that his choice is of grace and not of merit, and that Christ came into the world to save sinners, even the chief. Also, that the worthiness of Christ is of himself, and not from his ancestors. How little reason had the Jews, who were so called from this Judah, to boast as they did, Joh 8:41. What awful examples the Lord proclaims in his punishments, of his utter displeasure at sin! Let us seek grace from God to avoid every appearance of sin. And let that state of humbleness to which Jesus submitted, when he came to put away sin by the sacrifice of himself, in appointing such characters as those here recorded, to be his ancestors, endear the Redeemer to our hearts.And Onan knew that the seed should not be his,.... Should not be called a son of his, but a son of his brother Er; this is to be understood only of the firstborn; all the rest of the children born afterwards were reckoned the children of the real parent of them; this shows this was a custom in use in those times, and well known, and was not a peculiar case:

and it came to pass, when he went in unto his brother's wife; to cohabit with her, as man and wife, he having married her according to his father's direction:

that he spilled it on the ground, lest he should give his seed to his brother: lest his brother's wife he had married should conceive by him, and bear a son that should be called his brother's, and inherit his estate; and this is the sin, which from him is called Onania, a sin condemned by the light of nature, as well as by the word of God, and very prejudicial to mankind, as well as displeasing to God, as follows: