D&C 30

David Whitmer was one of the individuals blessed to be one of the three witnesses to the Book of Mormon.  One that not only saw and handled the gold plates but received the testimony of their truthfulness from the angel.  Not much time had passed from the glorius event that the Lord said to him -

"...you have feared man and have not relied on me for strength as you ought.
2 But your mind has been on the things of the earth more than on the things of me, your Maker, and the ministry whereunto you have been called; and you have not given heed unto my Spirit, and to those who were set over you, but have been persuaded by those whom I have not commanded.
3 Wherefore, you are left to inquire for yourself at my hand, and ponder upon the things which you have received."  (Doctrine and Covenants Section 30:1 - 3)

David Whitmer is not the only saint that has ever needed this counsel.  Many saints have received spiritual witnesses of the truth.  But it only seems to sustain them for a period unless they continually work, live, and give heed for and to the Spirit.  President Joseph Fielding Smith wrote -

"It seems a little strange that so soon after the wonderful manifestation which he [David Whitmer] had received and the witnessing of the outpouring of the Spirit at other times, that he would forget, but human nature is such that all of us need constant reminding of our responsibilities or we are likely to lapse into some indifference. The need of constant prayer and concentration of our thoughts on the things of the kingdom, and sincere attention to duty, is apparent with most of us, lest we slip. How frequently the Lord has had to caution his people against the weaknesses of the flesh!" (Joseph Fielding Smith, CHMR 1:146.)