74 But, corresponding to this love of God and of Christ, there must be love of the neighbor. How can we claim to love the Divine Redeemer, if we hate those whom He has redeemed with His precious blood, so that He might make them members of His Mystical Body? For that reason the beloved disciple warns us: "If any man say: 'I love God' and hates his brother, he is a liar. For he that loveth not his brother whom he seeth, how can he love God whom he seeth not? And this commandment we have from God, that he who loveth God loveth his brother also." Rather it should be said that the more we become "members one of another" "mutually careful, one for another,"the closer we shall be united with God and with Christ; as, on the other hand, the more ardent the love that binds us to God and to our divine Head, the closer we shall be united to each other in the bonds of charity.
Source: Mystici Corporis Christi (Vatican.va)