From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):

The relationship between man’s freedom and God’s law, which has its intimate and living centre in the moral conscience, is manifested and realized in human acts. It is precisely through his acts that man attains perfection as man, as one who is called to seek his Creator of his own accord and freely to arrive at full and blessed perfection by cleaving to him.¹¹⁹

Human acts are moral acts because they express and determine the goodness or evil of the individual who performs them.¹²⁰ They do not produce a change merely in the state of affairs outside of man but, to the extent that they are deliberate choices, they give moral definition to the very person who performs them, determining his profound spiritual traits. This was perceptively noted by Saint Gregory of Nyssa: “All things subject to change and to becoming never remain constant, but continually pass from one state to another, for better or worse . . . Now, human life is always subject to change; it needs to be born ever anew . . . But here birth does not come about by a foreign intervention, as is the case with bodily beings . . .; it is the result of a free choice. Thus we are in a certain way our own parents, creating ourselves as we will, by our decisions”.¹²¹

¹¹⁹ Cf. Second Vatican Ecumenical Council, Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 17.
¹²⁰ Cf. Saint Thomas Aquinas, Summa Theologiæ, I-II, q. 1, a. 3: “Idem sunt actus morales et actus humani.”
¹²¹ De Vita Moysis, II, 2-3: Patrologiæ Cursus completus, Series Græca 44, 327-328.

— Veritatis Splendor 71