18 This unity is, indeed, being attacked by false doctrines and by a variety of insidious strategems. But all should remember that the Mystical Body of Jesus Christ, the Church, must be "closely joined and knit together through every joint of the system according to the function in due measure of each single part," "until We all attain to the unity of the faith and of the deep knowledge of the Son of God, to perfect manhood, to the mature measure of the fullness of Christ," whose Vicar on earth is - by divine appointment - the Roman Pontiff, as successor of Peter.
Source: Meminisse Iuvat (Vatican.va)