D&C 121

There are many Christians who undoubtably believe in the teachings of the Bible with the assumption that anything beyond what is written there must be false.  Assuming that God cannot reveal additional truths and knowledge.  Even though He did exactly that for a period after His death and resurrection.

It was while experiencing great suffering in Liberty Jail, that the Lord told the Prophet -

"God shall give unto you knowledge by his Holy Spirit, yea, by the unspeakable gift of the Holy Ghost, that has not been revealed since the world was until now..."  (Doctrine and Covenants | Section 121:26)

And then speaking of those prophets that participated in much of the revelations and doctrines that are present in the Holy Bible, the Lord adds -

"Which our forefathers have awaited with anxious expectation to be revealed in the last times, which their minds were pointed to by the angels, as held in reserve for the fulness of their glory..."  (Doctrine and Covenants | Section 121:27)

Obviously, God did not reveal all truths and doctrines in previous dispensations which would also suggest that even now He has not revealed all for -

"We believe all that God has revealed, all that He does now reveal, and we believe that He will yet reveal many great and important things pertaining to the Kingdom of God."  (Pearl of Great Price | Articles of Faith 1:9)

We believe in this because the Lord tells us that -

"A time to come in the which nothing shall be withheld, whether there be one God or many gods, they shall be manifest.
29 All thrones and dominions, principalities and powers, shall be revealed and set forth upon all who have endured valiantly for the gospel of Jesus Christ.
30 And also, if there be bounds set to the heavens or to the seas, or to the dry land, or to the sun, moon, or stars—
31 All the times of their revolutions, all the appointed days, months, and years, and all the days of their days, months, and years, and all their glories, laws, and set times, shall be revealed in the days of the dispensation of the fulness of times—
32 According to that which was ordained in the midst of the Council of the Eternal God of all other gods before this world was, that should be reserved unto the finishing and the end thereof, when every man shall enter into his eternal presence and into his immortal rest."  (Doctrine and Covenants | Section 121:28 - 32)

Obviously, those that assume they have received all truth and knowledge are greatly mistaken.