The word mercy comes from a Greek word "Eusebeia" = It can be called "Devotion" Secular writers said that godliness is a correct attitude towards God and the things of God.
Piety then, are all those things that as Christians we can do to grow in our devotion to God.
Paul tells Timothy, "Do things that help develop your devotion to God, your reverence for God, your fear of God because those will bring you great rewards in this life and in the future life."
Exercising in piety means that we must work our spiritual muscle in order to have a greater devotion to God. But we must develop them like those who train in a gym.