Mystici Corporis Christi §69

⁶⁹ Now since its Founder willed this social body of Christ to be visible, the cooperation of all its members must also be externally manifest through their profession of the same faith and their sharing the same sacred rites, through participation in the same Sacrifice, and the practical observance of the same laws. Above all, it is absolutely necessary that the Supreme Head, that is, the Vicar of Jesus Christ on earth, be visible to the eyes of all, since it is He who gives effective direction to the work which all do in common in a mutually helpful way towards the attainment of the proposed end. As the Divine Redeemer sent the Paraclete, the Spirit of Truth, who in His name should govern the Church in an invisible way, so, in the same manner, He commissioned Peter and his successors to be His personal representatives on earth and to assume the visible government of the Christian community.