From the Catechism of the Catholic Church:
166 Faith is a personal act — the free response of the human person to the initiative of God who reveals himself. But faith is not an isolated act. No one can believe alone, just as no one can live alone. You have not given yourself faith as you have not given yourself life. The believer has received faith from others and should hand it on to others. Our love for Jesus and for our neighbor impels us to speak to others about our faith. Each believer is thus a link in the great chain of believers. I cannot believe without being carried by the faith of others, and by my faith I help support others in the faith.