D&C 112

Of the Quorum of the Twelve Apostles we learn -

"The twelve traveling councilors are called to be the Twelve Apostles, or special witnesses of the name of Christ in all the world—thus differing from other officers in the church in the duties of their calling.
24 And they form a quorum, equal in authority and power to the three presidents previously mentioned." (Doctrine and Covenants | Section 107:23 - 24)

Clearly the twelve apostles have the duty of the administration, preaching, teaching, and testifying of the Savior and His Church throughout the world. This calling is of such sacredness and importance that the Lord specifically planted in His vineyard (earth)-

"...twelve olive-trees;
45 And set watchmen round about them, and build a tower, that one may overlook the land round about, to be a watchman upon the tower, that mine olive-trees may not be broken down when the enemy shall come to spoil and take upon themselves the fruit of my vineyard." (Doctrine and Covenants | Section 101:44 - 45)

To the then president of the Quorum of the Twelve Apostles, Thomas B. Marsh, he provided a great revelation and counsel concerning his duties and the quorum's duties. First, it was importantant that Thomas understood that -

"Whosoever receiveth my word receiveth me, and whosoever receiveth me, receiveth those, the First Presidency, whom I have sent, whom I have made counselors for my name's sake unto you." (Doctrine and Covenants | Section 112:20)

As president of the Quorum of the Twelve, he was to follow the counsel of the First Presidency. And also in his administration -

"Exalt not yourselves; rebel not against my servant Joseph; for verily I say unto you, I am with him, and my hand shall be over him; and the keys which I have given unto him, and also to youward, shall not be taken from him till I come." (Doctrine and Covenants | Section 112:15)

At the time of this revelation, Thomas B. Marsh had received the keys of the kingdom. Had President remained faithful as so counseled in this revelation, he would have presided over the Church with the death of the Prophet and Hyrum Smith.