6 Our Lord Jesus Christ came down from Heaven for the very purpose of restoring amongst men the Kingdom of Peace, which the envy of the devil had destroyed, and it was His will that it should rest on no other foundation than that of brotherly love. These are His own oft-repeated words: "A new commandment I give unto you: That you love one another ( John xiv. 34); "This is my commandment that you love one another" ( John xv. 12); "These things I command you, that you love one another" ( John xv. 17); as though His one office and purpose was to bring men to mutual love. He used every kind of argument to bring about that effect. He bids us all look up to Heaven: "For one is your Father who is in Heaven" ( Matt . xxiii 9); He teaches all men, without distinction of nationality or of language, or of ideas, to pray in the words: "Our Father, who are in Heaven" ( Matt . vi. 9); nay, more, He tells us that our Heavenly Father in distributing the blessings of nature makes no distinction of our deserts: "Who maketh His sun to rise upon the good and bad, and raineth upon the just and the unjust" ( Matt . v. 45). He bids us be brothers one to another, and calls us His brethren: "All you are brethren" ( Matt . xxiii. 8); "that He might be the first-born amongst many brethren" ( Rom . vii. 29). In order the more to stimulate us to brotherly love, even towards those whom our natural pride despises, it is His will that we should recognize the dignity of His own very self in the meanest of men: "As long as you did it to one of these My least brethren, you did it to Me" ( Matt . xxv. 40}. At the close of His life did He not most earnestly beg of His Father, that as many as should believe in Him should all be one in the bond of charity? "As thou, Father, in Me, and I in Thee" ( John xvii. 21). And finally, as He was hanging from the cross, He poured out His blood over us all, whence being as it were compacted and fitly joined together in one body, we should love one another, with a love like that which one member bears to another in the same body.
Source: Ad Beatissimi Apostolorum (Vatican.va)