Objection 1. It seems that piety is not a special virtue distinct from other virtues. For the giving of service and homage to anyone proceeds from love. But it belongs to piety. Therefore piety is not a distinct virtue from charity.
Objection 2. Further, it is proper to religion to give worship to God. But piety also gives worship to God, according to Augustine (De Civ. Dei x). Therefore piety is not distinct from religion.
Objection 3. Further, piety, whereby we give our country worship and duty, seems to be the same as legal justice, which looks to the common good. But legal justice is a general virtue, according to the Philosopher (Ethic. v, 1,2). Therefore piety is not a special virtue.
On the contrary, It is accounted by Tully (De Invent. Rhet. ii) as a part of justice.
I answer that, A special virtue is one that regards an object under a special aspect. Since, then, the nature of justice consists in rendering another person his due, wherever there is a special aspect of something due to a person, there is a special virtue. Now a thing is indebted in a special way to that which is its connatural principle of being and government. And piety regards this principle, inasmuch as it pays duty and homage to our parents and country, and to those who are related thereto. Therefore piety is a special virtue.
Reply to Objection 1. Just as religion is a protestation of faith, hope and charity, whereby man is primarily directed to God, so again piety is a protestation of the charity we bear towards our parents and country.
Reply to Objection 2. God is the principle of our being and government in a far more excellent manner than one's father or country. Hence religion, which gives worship to God, is a distinct virtue from piety, which pays homage to our parents and country. But things relating to creatures are transferred to God as the summit of excellence and causality, as Dionysius says (Div. Nom. i): wherefore, by way of excellence, piety designates the worship of God, even as God, by way of excellence, is called "Our Father."
Reply to Objection 3. Piety extends to our country in so far as the latter is for us a principle of being: but legal justice regards the good of our country, considered as the common good: wherefore legal justice has more of the character of a general virtue than piety has.
Source: Summa Theologica (New Advent)