3 This world-wide and solemn testimony of allegiance and piety is especially appropriate to Jesus Christ, who is the Head and Supreme Lord of the race. His empire extends not only over Catholic nations and those who, having been duly washed in the waters of holy baptism, belong of right to the Church, although erroneous opinions keep them astray, or dissent from her teaching cuts them off from her care; it comprises also all those who are deprived of the Christian faith, so that the whole human race is most truly under the power of Jesus Christ. For He who is the Only-begotten Son of God the Father, having the same substance with Him and being the brightness of His glory and the figure of His substance (Hebrews i., 3) necessarily has everything in common with the Father, and therefore sovereign power over all things. This is why the Son of God thus speaks of Himself through the Prophet: "But I am appointed king by him over Sion, his holy mountain. . . The Lord said to me, Thou art my son, this day have I begotten thee. Ask of me and I will give thee the Gentiles for thy inheritance and the utmost parts of the earth for thy possession" (Psalm, ii.). By these words He declares that He has power from God over the whole Church, which is signified by Mount Sion, and also over the rest of the world to its uttermost ends. On what foundation this sovereign power rests is made sufficiently plain by the words, "Thou art My Son." For by the very fact that He is the Son of the King of all, He is also the heir of all His Father's power: hence the words-"I will give thee the Gentiles for thy inheritance," which are similar to those used by Paul the Apostle, "whom he bath appointed heir of all things" (Hebrews i., 2).
Source: Annum Sacrum (Vatican.va)