33 First, and most important of all, for mankind is the need of spiritual peace. We do not need a peace that will consist merely in acts of external or formal courtesy, but a peace which will penetrate the souls of men and which will unite, heal, and reopen their hearts to that mutual affection which is born of brotherly love. The peace of Christ is the only peace answering this description: "let the peace of Christ rejoice in your hearts." ( Colossians iii, 15) Nor is there any other peace possible than that which Christ gave to His disciples ( John xiv, 27) for since He is God, He "beholdeth the heart" (I Kings xvi, 7) and in our hearts His kingdom is set up. Again, Jesus Christ is perfectly justified when He calls this peace of soul His own for He was the first Who said to men, "all you are brethren." ( Matt . xxiii, 8) He gave likewise to us, sealing it with His own life's blood, the law of brotherly love, of mutual forbearance - "This is my commandment, that you love one another, as I have loved you." ( John xv, 12) "Bear ye one another's burdens; and so you shall fulfill the law of Christ." ( Galatians vi, 2)
Source: Ubi Arcano Dei Consilio (Vatican.va)