It is true that God has made his entire people a royal priesthood in Christ.  But our High Priest, Jesus Christ, also chose some of his followers to carry out publicly in the Church a priestly ministry in his name on behalf of mankind.  He was sent by the Father, and he in turn sent the apostles into the world; through them and their successors, the bishops, he continues his work as Teacher, Priest, and Shepherd.  Priests are co-workers of the order of bishops.  They are joined to the bishops in the priestly office and are called to serve God's people.