24 God, Who has fatherly concern for everyone, has willed that all men should constitute one family and treat one another in a spirit of brotherhood. For having been created in the image of God, Who "from one man has created the whole human race and made them live all over the face of the earth" (Acts 17:26), all men are called to one and the same goal, namely God Himself. For this reason, love for God and neighbor is the first and greatest commandment. Sacred Scripture, however, teaches us that the love of God cannot be separated from love of neighbor: "If there is any other commandment, it is summed up in this saying: Thou shalt love thy neighbor as thyself.... Love therefore is the fulfillment of the Law" (Rom. 13:9-10; cf. 1 John 4:20). To men growing daily more dependent on one another, and to a world becoming more unified every day, this truth proves to be of paramount importance. Indeed, the Lord Jesus, when He prayed to the Father, "that all may be one. . . as we are one" (John 17:21-22) opened up vistas closed to human reason, for He implied a certain likeness between the union of the divine Persons, and the unity of God's sons in truth and charity. This likeness reveals that man, who is the only creature on earth which God willed for itself, cannot fully find himself except through a sincere gift of himself.
Source: Gaudium et Spes (Vatican.va)