Why is the Catholic Church one? The Catholic Church is one because all its members, according to the will of Christ, profess the same faith, have the same sacrifice and sacraments, and are united under one and the same visible head, the Pope.
Because the bread is one, we though many, are one body, all of us who partake of the one bread. (I Corinthians 10:17)
Further reading: CCC 813-822