One of the Gifts of the Holy Spirit:
Piety is one of the seven gifts of the Holy Spirit enumerated in Isaiah 11:2-3. We receive the seven gifts of the Holy Spirit when we are infused with sanctifying grace, the life of God within us—as, for example, when we receive a sacrament worthily. As the current Catechism of the Catholic Church notes, "They complete and perfect the virtues of those who receive them." When we are infused with the gifts of the Holy Spirit, we respond to the promptings of the Holy Spirit as if by instinct, the way that Christ Himself would.
The Perfection of Religion:
Perhaps in none of the gifts of the Holy Spirit is this instinctual response more obvious than in piety. While wisdom and knowledge perfect the theological virtue of faith, piety perfects religion, which, as Fr. John A. Hardon, S.J., notes in his Modern Catholic Dictionary, is "The moral virtue by which a person is disposed to render to God the worship and service he deserves." Far from being a drudgery, worship should be an act of love, and piety is the instinctive affection for God that makes us desire to render worship to Him, just as we voluntarily honor our parents.
Piety in Practice:
Piety, Father Hardon notes, arises "not so much from a studied effort or acquired habit as from a supernatural communication conferred by the Holy Spirit." People sometimes say that "piety demands it," which usually means that they feel compelled to do something that they don't want to do. But true piety makes no such demands but instills in us a desire always to do that which is pleasing to God (and, by extension, that which is pleasing to those who serve God in their own lives).