D&C 18

Having authority as newly ordained apostles, Oliver Cowdery and David Whitmer are instructed of the Lord -

"...I give unto you, Oliver Cowdery, and also unto David Whitmer, that you shall search out the Twelve, who shall have the desires of which I have spoken;
38 And by their desires and their works you shall know them."  (Doctrine and Covenants  Section 18:37 - 38)

Martin Harris was later added to this group.  To understand the qualifications for this calling, the Lord teaches some important principles being -

"Remember the worth of souls is great in the sight of God..."  (Doctrine and Covenants
Section 18:10)   Which would suggest that in the eyes of God, everying single individual has is loved and is of great worth to God.  Even such -   "...how great is his joy in the soul that repenteth!"  (Doctrine and Covenants  Section 18:13)   Note the exclamation point.  I find that quite meaningful that as we serve and declare repentance that the Lord experiences great joy as each individual is willing to repent and come to Him.  And also -   "...if it so be that you should labor all your days in crying repentance unto this people, and bring, save it be one soul unto me, how great shall be your joy with him in the kingdom of my Father! 16 And now, if your joy will be great with one soul that you have brought unto me into the kingdom of my Father, how great will be your joy if you should bring many souls unto me!
17 Behold, you have my gospel before you, and my rock, and my salvation."  (Doctrine and Covenants  Section 18:15 - 17)

It is the gospel message that brings such great joy.  Though much of the revelation is instructions for the members of the Quorum of the Twelve Apostles, I believe the principles have great application to every saint as the instructions continue -

"...now I speak unto you, the Twelve—Behold, my grace is sufficient for you; you must walk uprightly before me and sin not.
32 And, behold, you are they who are ordained of me to ordain priests and teachers; to declare my gospel, according to the power of the Holy Ghost which is in you, and according to the callings and gifts of God unto men..."  (Doctrine and Covenants   Section 18:31 - 32)

I note the strong words of "sin not" for -

"...you shall fall down and worship the Father in my name.
41 And you must preach unto the world, saying: You must repent and be baptized, in the name of Jesus Christ;
42 For all men must repent and be baptized, and not only men, but women, and children who have arrived at the years of accountability.
43 And now, after that you have received this, you must keep my commandments in all things..."  (Doctrine and Covenants  Section 18:40 - 43)

With the warning -

"...after that you have received this, if you keep not my commandments you cannot be saved in the kingdom of my Father."  (Doctrine and Covenants   Section 18:46)

There is another principle that I believe is of great importance that we should understand is that -

"...I, Jesus Christ, your Lord and your God, have spoken it.
34 These words are not of men nor of man, but of me; wherefore, you shall testify they are of me and not of man;
35 For it is my voice which speaketh them unto you; for they are given by my Spirit unto you, and by my power you can read them one to another; and save it were by my power you could not have them;
36 Wherefore, you can testify that you have heard my voice, and know my words."  (Doctrine and Covenants   Section 18:33 - 36)

By that definition, I can testify that I have heard His voice.