71 "One Lord, one faith," writes the Apostle: the faith, that is, by which we hold fast to God, and to Jesus Christ whom He has sent. The beloved disciple teaches us how closely this faith binds us to God: "Whosoever shall confess that Jesus is the Son of God, God abideth in him, and he in God." This Christian faith binds us no less closely to each other and to our divine Head. For all we who believe, "having the same spirit of faith," are illumined by the same light of Christ, nourished by the same Food of Christ, and live under the teaching authority of Christ. If the same spirit of faith breathes in all, we are all living the same life "in the faith of the Son of God who loved us and delivered himself for us." And once we have received Christ, our Head, through an ardent faith so that He dwells within our hearts, as He is the author so He will be the finisher of our faith.
Source: Mystici Corporis Christi (Vatican.va)