D&C 18

To Oliver Cowdery, the Lord said -

"...I have manifested unto you, by my Spirit in many instances, that the things which you have written are true; wherefore you know that they are true.
3 And if you know that they are true, behold, I give unto you a commandment, that you rely upon the things which are written..." (Doctrine and Covenants Section 18:2 - 3)

At this time, Oliver Cowdery had already had the hands of an angel laid upon his head and even heard the voice of the Lord. He had seen the plates at the hands of an angel and yet the Lord emphasizes the manifestation of His Spirit to be that by which Oliver knows that what is written is true. He is commanded to "rely upon the things which are written". To rely, one must read and ponder that which is written. Of the written words, the Savior said -

"...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..." (Doctrine and Covenants Section 18:33 - 34)

- because the Spirit does manifest it. Then the Lord provides a profound reasoning to all that read and ponder these words -

"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 18:35 - 36)

Elder S. Dilworth Young said regarding these verses -

“In 1835 the Twelve were chosen, as you know, and on one occasion they were called together and given their instructions. Oliver Cowdery was the spokesman; and after having given them some very powerful and heartwarming instruction, so moved was he, himself, that he had to stop two or three times to weep. He finally read the revelation [now designated as section 18].

Brigham Young was so impressed by it that he copied it in his laborious handwriting into his diary. I am impressed by it likewise. These are the words: [D&C 18:34–36].

The thing that impresses me about this is, and I have never thought of it before, when I read a verse in the Doctrine and Covenants I am hearing the voice of the Lord as well as reading his words, if I hear by the Spirit.

Now I have heard it said many times by men that they have often asked the Lord for a special testimony and oftentimes haven’t had it. They seem to want to hear the voice of the Lord. I confess I have often wanted to hear the voice of the Lord, without knowing that all these years I have been hearing it with deaf ears. This woke me up.” (In Conference Report, Apr. 1963, p. 74.)

I too have felt His Spirit testify to me of their truth. Therefore, I can testify that I have heard His voice and know His words.