Joseph Smith taught that a knowledge of the existence of God’s truthfulness is necessary:
“And lastly, but not less important to the exercise of faith in God, is the idea of the existence of the attribute truth in him; for without the idea of the existence of this attribute the mind of man could have nothing upon which it could rest with certainty—all would be confusion and doubt. But with the idea of the existence of this attribute in the Deity in the mind, all the teachings, instructions, promises, and blessings, become realities, and the mind is enabled to lay hold of them with certainty and confidence, believing that these things, and all that the Lord has said, shall be fulfilled in their time; and that all the cursings, denunciations, and judgments, pronounced upon the heads of the unrighteous, will also be executed in the due time of the Lord: and, by reason of the truth and veracity of him, the mind beholds its deliverance and salvation as being certain”
(Lectures on Faith 4:16).