Objection 1. It seems that piety does not extend to particular human individuals. For Augustine says (De Civ. Dei x) that piety denotes, properly speaking, the worship of God, which the Greeks designate by the term eusebeia. But the worship of God does not denote relation to man, but only to God. Therefore piety does not extend definitely to certain human individuals.
Objection 2. Further, Gregory says (Moral. i): "Piety, on her day, provides a banquet, because she fills the inmost recesses of the heart with works of mercy." Now the works of mercy are to be done to all, according to Augustine (De Doctr. Christ. i). Therefore piety does not extend definitely to certain special persons.
Objection 3. Further, in human affairs there are many other mutual relations besides those of kindred and citizenship, as the Philosopher states (Ethic. viii, 11,12), and on each of them is founded a kind of friendship, which would seem to be the virtue of piety, according to a gloss on 2 Timothy 3:5, "Having an appearance indeed of piety [Douay: 'godliness']." Therefore piety extends not only to one's kindred and fellow-citizens.
On the contrary, Tully says (De Invent. Rhet. ii) that "it is by piety that we do our duty towards our kindred and well-wishers of our country and render them faithful service."
I answer that, Man becomes a debtor to other men in various ways, according to their various excellence and the various benefits received from them. on both counts God holds first place, for He is supremely excellent, and is for us the first principle of being and government. On the second place, the principles of our being and government are our parents and our country, that have given us birth and nourishment. Consequently man is debtor chiefly to his parents and his country, after God. Wherefore just as it belongs to religion to give worship to God, so does it belong to piety, in the second place, to give worship to one's parents and one's country.
The worship due to our parents includes the worship given to all our kindred, since our kinsfolk are those who descend from the same parents, according to the Philosopher (Ethic. viii, 12). The worship given to our country includes homage to all our fellow-citizens and to all the friends of our country. Therefore piety extends chiefly to these.
Reply to Objection 1. The greater includes the lesser: wherefore the worship due to God includes the worship due to our parents as a particular. Hence it is written (Malachi 1:6): "If I be a father, where is My honor?" Consequently the term piety extends also to the divine worship.
Reply to Objection 2. As Augustine says (De Civ. Dei x), "the term piety is often used in connection with works of mercy, in the language of the common people; the reason for which I consider to be the fact that God Himself has declared that these works are more pleasing to Him than sacrifices. This custom has led to the application of the word 'pious' to God Himself."
Reply to Objection 3. The relations of a man with his kindred and fellow-citizens are more referable to the principles of his being than other relations: wherefore the term piety is more applicable to them.
Source: Summa Theologica (New Advent)