1 If one be found slain in the land which the LORD thy God giveth thee to possess it, lying in the field, and it be not known who hath slain him: 2 Then thy elders and thy judges shall come forth, and they shall measure unto the cities which are round about him that is slain: 3 And it shall be, that the city which is next unto the slain man, even the elders of that city shall take an heifer, which hath not been wrought with, and which hath not drawn in the yoke; 4 And the elders of that city shall bring down the heifer unto a rough valley, which is neither eared nor sown, and shall strike off the heifer's neck there in the valley: 5 And the priests the sons of Levi shall come near; for them the LORD thy God hath chosen to minister unto him, and to bless in the name of the LORD; and by their word shall every controversy and every stroke be tried: 6 And all the elders of that city, that are next unto the slain man, shall wash their hands over the heifer that is beheaded in the valley: 7 And they shall answer and say, Our hands have not shed this blood, neither have our eyes seen it. 8 Be merciful, O LORD, unto thy people Israel, whom thou hast redeemed, and lay not innocent blood unto thy people of Israel's charge. And the blood shall be forgiven them. 9 So shalt thou put away the guilt of innocent blood from among you, when thou shalt do that which is right in the sight of the LORD. 10 When thou goest forth to war against thine enemies, and the LORD thy God hath delivered them into thine hands, and thou hast taken them captive, 11 And seest among the captives a beautiful woman, and hast a desire unto her, that thou wouldest have her to thy wife; 12 Then thou shalt bring her home to thine house; and she shall shave her head, and pare her nails; 13 And she shall put the raiment of her captivity from off her, and shall remain in thine house, and bewail her father and her mother a full month: and after that thou shalt go in unto her, and be her husband, and she shall be thy wife. 14 And it shall be, if thou have no delight in her, then thou shalt let her go whither she will; but thou shalt not sell her at all for money, thou shalt not make merchandise of her, because thou hast humbled her. 15 If a man have two wives, one beloved, and another hated, and they have born him children, both the beloved and the hated; and if the firstborn son be hers that was hated: 16 Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn: 17 But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his. 18 If a man have a stubborn and rebellious son, which will not obey the voice of his father, or the voice of his mother, and that, when they have chastened him, will not hearken unto them: 19 Then shall his father and his mother lay hold on him, and bring him out unto the elders of his city, and unto the gate of his place; 20 And they shall say unto the elders of his city, This our son is stubborn and rebellious, he will not obey our voice; he is a glutton, and a drunkard. 21 And all the men of his city shall stone him with stones, that he die: so shalt thou put evil away from among you; and all Israel shall hear, and fear. 22 And if a man have committed a sin worthy of death, and he be to be put to death, and thou hang him on a tree: 23 His body shall not remain all night upon the tree, but thou shalt in any wise bury him that day; (for he that is hanged is accursed of God;) that thy land be not defiled, which the LORD thy God giveth thee for an inheritance.
[AD 253] Origen of Alexandria on Deuteronomy 21:10-11
But nevertheless I also intellectually have “gone out to war against my enemies, and I saw there” in the plunder “a woman with a beautiful figure.” Whatever we find said well and reasonably among our philosophical enemies, or we read anything said among them wisely and knowingly, we must cleanse it. We must remove and cut off all that is dead and worthless. It is as if one were trimming the hairs of the head and the nails of the woman taken from the spoils of the enemy. Only then would you take her as a wife.

[AD 215] Clement of Alexandria on Deuteronomy 21:13
The [Deuteronomic] law wishes males to have responsible sexual relations with their marriage partners, solely for the generation of children. This is clear when a bachelor is prevented from enjoying immediate sexual relations with a woman prisoner of war. If he once falls in love with her, he must let her cut her hair short and mourn for thirty days. If even so his desire has not faded away, then he may father children by her. The fixed period of time enables the overpowering impulse to be scrutinized and to turn into a more rational appetency.

[AD 397] Ambrose of Milan on Deuteronomy 21:15
Since that discourse took up an example from Deuteronomy for its assertion, where it is connected with “the man who had two wives, one hateful, the other lovable,” it is not unreasonable for you to be concerned, lest someone should perhaps think that the passage advocated that the man had two souls. This is not at all possible.Truly, you yourself are not unaware that sometimes when Scripture speaks allegorically, one thing refers to the form of the synagogue, another to the church. One thing refers to the soul, another to the mystery of the Word and another to different forms and qualities of souls which the spiritually discerning person recognizes. In the following chapter of the law I judge that not two souls but different qualities of one soul were meant. For there is the form of the lovable soul, which delights in pleasures, flees from labor, shuns compunction and rejects the judgment of God. It is lovable in this way: it seems sweet and agreeable at the time. This form of the soul does not affect the mind but diverts it. But that other form of the soul is rather sadder. It is perfected by zeal for God, just as the severe wife is unwilling to allow her spouse to go to harlots. She does not endure it. She simply does not allow it. This form of the soul does not indulge the body in any way; she yields to no pleasures or delights. She rejects shameful secrets; she pursues harsh labors and grave dangers.

[AD 397] Ambrose of Milan on Deuteronomy 21:17
So the digression we made from one law to another was not pointless, so that we teach that the firstborn is not of that lovable wife, that is, the son of the wife who is indulgent and devoted to pleasure, although the verses before us express this thought. The words of Scripture say, “He shall not propose the son of his lovable wife as the firstborn when he knows that the son of his hateful wife is the firstborn.” Rather, he who is the holy offspring of a holy mother is genuinely the firstborn; true sons do not stray from their true mother though sinners do. Therefore, he is not the true firstborn who is not the son of the true mother, but like a firstborn he is helped by richers and honored lest he be in need. But the firstborn receives “a double portion of all that he has” in order that he may be rich, [just as] in Genesis you find that each patriarch received a gift of two cloaks from their brother Joseph when they were sent back to their father. [This signified] that their brother Joseph, who their father believed to be dead, was found.

[AD 50] Galatians on Deuteronomy 21:23
O foolish Galatians, who hath bewitched you, that ye should not obey the truth, before whose eyes Jesus Christ hath been evidently set forth, crucified among you? This only would I learn of you, Received ye the Spirit by the works of the law, or by the hearing of faith? Are ye so foolish? having begun in the Spirit, are ye now made perfect by the flesh? Have ye suffered so many things in vain? if it be yet in vain. He therefore that ministereth to you the Spirit, and worketh miracles among you, doeth he it by the works of the law, or by the hearing of faith? Even as Abraham believed God, and it was accounted to him for righteousness. Know ye therefore that they which are of faith, the same are the children of Abraham. And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed. So then they which be of faith are blessed with faithful Abraham. For as many as are of the works of the law are under the curse: for it is written, Cursed is every one that continueth not in all things which are written in the book of the law to do them. But that no man is justified by the law in the sight of God, it is evident: for, The just shall live by faith. And the law is not of faith: but, The man that doeth them shall live in them. Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree: [Deuteronomy 21:23] That the blessing of Abraham might come on the Gentiles through Jesus Christ; that we might receive the promise of the Spirit through faith.
[AD 373] Athanasius of Alexandria on Deuteronomy 21:23
If any of our own people also inquire, not from love of debate but from love of learning, why he suffered death in no other way except on the cross, let him also be told that no other way than this was good for us and that it was well that the Lord suffered this for our sakes. For if he himself came to bear the curse laid upon us, how else could he have “become a curse” unless he received the death set for a curse? And that is the cross. For this is exactly what is written: “Cursed is he that hangs on a tree.”