Catechism of the Catholic Church §940 — excerpt
"The characteristic of the lay state being a life led in the midst of the world and of secular affairs, lay people are called by God to make of their apostolate, through the vigor of their Christian spirit, a leaven in the world" (AA 2 # 2).
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.