Without the feelings of humanity are they who have put off the first affections of nature towards their own relations. As he mentions the want of mercy as an evidence of human nature being depraved, Augustine, in arguing against the Stoics, concludes, that mercy is a Christian virtue.

Without understanding - Inconsiderate, or foolish; see Romans 1:21-22.
Covenant breakers - Perfidious; false to their contracts.
Without natural affections - This expression denotes the lack of affectionate regard toward their children. The attachment of parents to children is one of the strongest in nature, and nothing can overcome it but the most confirmed and established wickedness. And yet the apostle charges on the pagan generally the lack of this affection. He doubtless refers here to the practice so common among pagans of exposing their children, or putting them to death. This crime, so abhorrent to all the feelings of humanity, was common among the pagan, and is still. The Canaanites, we are told Psalm 106:37-38, "sacrificed their sons and their daughters unto devils, and shed innocent blood, even the blood of their sons and their daughters, whom they sacrificed unto the idols of Canaan." Manasseh among the Jews imitated their example, and introduced the horrid custom of sacrificing children to Moloch, and set the example by offering his own; 2-Chronicles 33:6.
Among the ancient Persians it was a common custom to bury children alive. In most of the Grecian states, infanticide was not merely permitted, but actually enforced by law. The Spartan lawgiver expressly ordained that every child that was born should be examined by the ancient men of the tribe, and that if found weak or deformed, should be thrown into a deep cavern at the foot of Mount Taygetus. Aristotle, in his work on government, enjoins the exposure of children that are naturally feeble and deformed, in order to prevent an excess of population. But among all the nations of antiquity, the Romans were the most unrelenting in their treatment of infants. Romulus obliged the citizens to bring up all their male children, and the oldest of the females, proof that the others were to be destroyed. The Roman father had an absolute right over the life of his child, and we have abundant proof that that right was often exercised.
Romulus expressly authorized the destruction of all children that were deformed, only requiring the parents to exhibit them to their five nearest neighbors, and to obtain their consent to their death. The law of the Twelve Tables enacted in the 301st year of Rome, sanctioned the same barbarous practice. Minucius Felix thus describes the barbarity of the Romans in this respect: "I see you exposing your infants to wild beasts and birds, or strangling them after the most miserable manner." (chapter xxx.) Pliny the older defends the right of parents to destroy their children, upon the ground of its being necessary in order to preserve the population within proper bounds. Tertullian, in his apology, expresses himself boldly on this subject. "How many of you (addressing himself to the Roman people, and to the governors of cities and provinces) might I deservedly charge with infant murder; and not only so, but among the different kinds of death, for choosing some of the cruelest for their own children, such as drowning, or starving with cold or hunger, or exposing to the mercy of dogs; dying by the sword being too sweet a death for children."
Nor was this practice arrested in the Roman government until the time of Constantine, the first Christian prince. The Phoenicians and Carthaginians were in the habit of sacrificing infants to the gods. It may be added that the crime is no less common among modern pagan nations. No less than 9000 children are exposed in Pekin in China annually. Persons are employed by the police to go through the city with carts every morning to pick up all the children that may have been thrown out during the night. The bodies are carried to a common pit without the walls of the city, into which all, whether dead or living, are promiscuously thrown. (Barrow's Travels in China, p. 113, Amos ed.) Among the Hindus the practice is perhaps still more common. In the provinces of Cutch and Guzerat alone the number of infantile murders amounted, according to the lowest calculation in 1807, to 3,000 annually; according to another calculation, to 30,000.
Females are almost the only victims. (Buchanan's Researches in Asia, Eng. ed. p. 49. Ward's View of the Hindus.) In Otaheite, previously to the conversion of the people to Christianity. it was estimated that at least two-thirds of the children were destroyed. (Turnbull's Voyage round the World in 1800, 2, 3, and 4.) The natives of New South Wales were in the habit of burying the child with its mother, if she should happen to die. (Collins' Account of the Colony of New South Wales, p. 124, 125.) Among the Hottentots, infanticide is a common crime. "The altars of the Mexicans were continually drenched in the blood of infants." In Peru, no less than two hundred infants were sacrificed on occasion of the coronation of the Inca. The authority for these melancholy statements may be seen in Beck's Medical Jurisprudence, vol. i. 18-197, ed. 1823; see also Robertson's History of America, p. 221, ed. 1821. This is a specimen of the views and feelings of the pagan world; and the painful narrative might be continued to almost any length. After this statement, it cannot surely be deemed a groundless charge when the apostle accused them of being destitute of natural affection.
Implacable - This word properly denotes those who will not be reconciled where there is a quarrel; or who pursue the offender with unyielding revenge. It denotes an unforgiving temper; and was doubtless common among the ancients, as it is among all pagan people. The aborigines of America have given the most striking manifestation of this that the world has known. It is well known that among them, neither time nor distance will obliterate the memory of an offence; and that the avenger will pursue the offender over hills and streams, and through heat or snow, happy if he may at last, though at the expiration of years, bury the tomahawk in the head of his victim, though it may be at the expense of his own life. See Robertson's America, book iv. Section lxxiii. - lxxxi.
Unmerciful - Destitute of compassion. As a proof of this, we may remark that no provisions for the poor or the infirm were made among the pagan. The sick and the infirm were cast out, and doomed to depend on the stinted charity of individuals. Pure religion, only, opens the heart to the appeals of want; and nothing but Christianity has yet expanded the hearts of people to make public provisions for the poor, the ignorant, and the afflicted.

Without understanding - Ασυνετους, from α, negative, and συνετος, knowing; persons incapable of comprehending what was spoken; destitute of capacity for spiritual things.
Covenant-breakers - Ασυνθετους, from α, negative, and συντιθημυι, to make an agreement; persons who could be bound by no oath, because, properly speaking, they had no God to witness or avenge their misconduct. As every covenant, or agreement, is made as in the presence of God, so he that opposes the being and doctrine of God is incapable of being bound by any covenant; he can give no pledge for his conduct.
Without natural affection - Αστοργους; without that attachment which nature teaches the young of all animals to have to their mothers, and the mothers to have for their young. The heathens, in general, have made no scruple to expose the children they did not think proper to bring up, and to despatch their parents when they were grown old or past labor.
Implacable - Ασπονδους, from α, negative; and σπονδη, A Libation. It was customary among all nations to pour out wine as a libation to their gods, when making a treaty. This was done to appease the angry gods, and reconcile them to the contracting parties. The word here shows a deadly enmity; the highest pitch of an unforgiving spirit; in a word, persons who would not make reconciliation either to God or man.
Unmerciful - Ανελεημονας; those who were incapable, through the deep-rooted wickedness of their own nature, of showing mercy to an enemy when brought under their power, or doing any thing for the necessitous, from the principle of benevolence or commiseration.

Without understanding, (n) covenantbreakers, without natural affection, implacable, unmerciful:
(n) Not caring if they keep their covenants and bargains.

Without understanding,.... Of God, of his nature and worship, of things divine and even moral, being given up to a reprobate mind:
covenant breakers; had no regard to private or public contracts:
without natural affection; to their parents, children, relations and friends:
implacable; when once offended there was no reconciling of them:
unmerciful; had no pity and compassion to persons in distress.

Covenant - breakers - It is well known, the Romans, as a nation, from the very beginning of their commonwealth, never made any scruple of vacating altogether the most solemn engagement, if they did not like it, though made by their supreme magistrate, in the name of the whole people. They only gave up the general who had made it, and then supposed themselves to be at full liberty. Without natural affection - The custom of exposing their own new - born children to perish by cold, hunger, or wild beasts, which so generally prevailed in the heathen world, particularly among the Greeks and Romans, was an amazing instance of this; as is also that of killing their aged and helpless parents, now common among the American heathens.