The Catholic Church is missionary because every member shares the command from Christ to carry the Good News of His teaching to all mankind by word and example.
The Church, as a community of believers and brotherly love, bears Christian witness by its preaching of the Gospel and its service to others.
God has called to be witnesses all those who believe He has revealed and given Himself to men in Jesus Christ. The Church gives this witness by proclaiming in the world, by word and deed, what God has done in Jesus Christ. The Church must live the life of Jesus in His Spirit and show His love, by her life of brotherhood and service to others.
Jesus said to the Apostles, “All authority in heaven and on earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you; and lo, I am with you always, to the close of the age” (Matthew 28:18-20).
In reality, the Church is its members. The Christian community should follow the example of its founder, Jesus Christ—serving the larger human community by sharing the riches of its faith and reaching out to help all those in need. Christian witness, or missionary activity, gives testimony to the loving presence of God in the world.
St. Luke says of the first Christians, “And they devoted themselves to the apostles’ teaching and fellowship, to the breaking of bread and the prayers” (Acts 2:42).