For, putting the hand under the thigh of a man and swearing by the God of heaven, what else did that signify except that in that flesh, which took its origin from that thigh, the God of heaven would come? Marriage therefore is a good in which the married are better in proportion as they fear God more chastely and more faithfully, especially if they also nourish spiritually the children whom they desire carnally.
For an important thing was being done when a spouse was being sought for the seed of Abraham. But that the servant might learn this which Abraham knew, that he did not desire grandchildren carnally and that he did not have any carnal conception about his progeny, he said to his slave whom he was sending, “Put your hand under my thigh and swear by the God of heaven.” What does the God of heaven want to signify in respect to the thigh of Abraham? Already you understand the hidden meaning: by the thigh, his progeny. Therefore what was that swearing but a signifying that the God of heaven would come in the flesh from the progeny of Abraham? Tractate on the Gospel of John.
When the sacred lesson was read a little while ago, we heard that blessed Abraham called his servant and said to him, “Put your hand under my thigh that I may adjure you by the God of heaven and of earth, not to obtain a wife for my son from the daughters of this region.” And [the servant] obediently placed his hand under his thigh and swore to him. Indeed, brothers, all these things that are read in the Old Testament, if we are willing to accept them only according to the letter, will bring us little or no profit of soul. For of what benefit is it to us who assemble in church with devotion to hear the Word of God, if it is mentioned that Abraham sent his servant to bring his son a wife from a distant country, when we see this happen frequently also in this land? However, brothers, following the blessed apostle Paul, we should believe that all things which were written for the Jews “happened to them as a type” but in reality were fulfilled for us. Therefore Abraham said to his servant,...
Abraham made him swear by the covenant of circumcision. Because God saw that the two heads of the world had dishonored this member, he set the sign of the covenant on it so that this member, which was the most despised of all the limbs, would now be the most honored of all the limbs. The sign of the covenant that was set on it bestowed such great honor that those who take oaths now swear by it and all those who administer oaths make them swear by it.
Servant. Eliezer, or Damascus, whom he had once intended for his heir, chap. xv. 2. (Haydock)
Under either to show their subjection, (Sa.) or their faith in Christ, who should be born of Abraham, (St. Jerome, ep. 140) or to testify that their oath shall be no less binding than the covenant of circumcision. For this last reason, the Jews still observe the custom of sitting upon the hand of the person who takes an oath. (Menochius) See chap. xlvii. 29, where Jacob imitates the action of his grand-father. These two patriarchs, progenitors of Christ, are the only ones in Scripture whom we find practising it; whence St. Augustine and St. Ambrose conclude, that it had a reference to the mysterious birth of our Redeemer. (Bonfrere.)
You see, since he had reached extreme old age, the text says, he wished to preserve Isaac from association with the Canaanites, lest he take a wife from among them. So he summoned the more prudent of his servants, the text says, and gave him the following instructions: “Place your hand under my thigh.” In Greek the verse is written this way: “under my thigh”; whereas in Hebrew it says “under my loins.” Why did he speak in this fashion? It was an idiom of people in the past. But on other grounds it was also because the birth of Isaac takes its origin from there. For you to learn that the action was done according to a certain custom, notice that when he was ordering him to put his hand there, he immediately added, “and I will make you swear by the Lord, the God of heaven and the God of earth.” See how he teaches the servant to recognize the Creator of all things. By saying, “the God of heaven and the God of earth,” he encompassed all creation.