In an 1837 revelation to Thomas B. Marsh, then president of the quorum, He said the Twelve are "chosen to bear testimony of my name and to send it abroad among all nations, kindreds, tongues, and people," (Doctrine and Covenants 112:1), that they are to send forth His word "unto the ends of the earth" (verse 4), feed His sheep (verse 14) and "unlock the door of the kingdom in all places" where the First Presidency cannot come (verse 17).
President Boyd K. Packer, President of the Quorum of the Twelve, told the Church News, "The scriptures describe the Twelve as 'a Traveling Presiding High Council, to officiate in the name of the Lord, under the direction of the Presidency of the Church . . . ; to build up the church, and regulate all the affairs of the same in all nations" (Doctrine and Covenants 107:33). The Twelve are 'to call upon the Seventy, when they need assistance, to fill the several calls for preaching and administering the gospel, instead of any others' (Doctrine and Covenants 107:38). The Lord has promised that He would reveal the way to fulfill these assignments as the need came.
"I remember when not too many years ago it could be said that every stake president in the Church had shaken hands with a member of the Twelve in the past year. Now that the Church has grown and spread abroad, as it was prophesied, we have organized a system that allows us to communicate with many stakes in a single assignment. In this way the world stays small enough so that we can keep contact with the members, particularly the priesthood leaders."