God has promised through His holy servants that if we, His sons and daughters, keep His commandments and have faith on His Name that the eternal purposes of Heaven shall all be made known unto us. The strong in spirit shall become stronger, and the weak and those who do wickedly shall become weaker. The strong, as is said, “Shall go on from faith to faith,” until all the purposes of God are fulfilled.
The first verse written in the Book of Mormon refers to the Mysteries of God
I, Nephi, having been born of goodly parents, therefore I was taught somewhat in all the learning of my father; and having seen many afflictions in the course of my days, nevertheless, having been highly favored of the Lord in all my days; yea, having had a great knowledge of the goodness and the Mysteries of God, therefore I make a record of my proceedings in my days. (I Nephi 1:1)
Later Nephi said:
And it came to pass that I, Nephi, being exceeding young, nevertheless being large in stature, and also having great desires to know of the Mysteries of God, wherefore, I did cry unto the Lord; and behold He did visit me, and did soften my heart that I did believe all the words which had been spoken by my father; wherefore, I did not rebel against him like unto my brothers. (Ibid., 2:16)
Again Nephi said:
For he that diligently seeketh shall find; and the Mysteries of God shall be unfolded unto them, by the power of the Holy Ghost, as well in these times as in times of old, and as well in times of old as in times to come; wherefore, the course of the Lord is one eternal round. (Ibid., 10:19)
When King Benjamin called all his people to meet in a conference which was held in Zarahemla, 125 years B.C., among other things he said, he being the High Priest of the Church or its President:
My brethren, all ye that have assembled yourselves together, you that can hear my words which I shall speak unto you this day; for I have not commanded you to come up hither to trifle with the words which I shall speak, but that you should hearken unto me, and open your ears that ye may hear, and your hearts that ye may understand, and your minds that the Mysteries of God may be unfolded to your view. (Mosiah 2:9)
When Alma grew old and appointed his son, Helaman, to succeed him in the high office he then held, among the instructions Alma gave unto Helaman were these:
Now ye may suppose that this is foolishness in me; but behold I say unto you, that by small and simple things are great things brought to pass; and small means in many instances doth confound the wise.
And the Lord God doth work by means to bring about His great and eternal purposes; and by very small means the Lord doth confound the wise and bringeth about the Salvation of many souls.
And now, it has hitherto been wisdom in God that these things should be preserved; for behold, they have enlarged the memory of this people, yea, and convinced many of the error of their ways, and brought them to a knowledge of their God unto the Salvation of their souls.
Yea, I say unto you, were it not for these things that these records do contain, which are upon these plates, Ammon and his brethren could not have convinced so many thousands of the Lamanites of the incorrect traditions of their fathers; yea, these records and their words brought them unto repentance; that is, they brought them to a knowledge of the Lord their God, and to rejoice in Jesus Christ their Redeemer.
And who knoweth but what they will be the means of bringing many thousands of them, yea, and also many thousands of our stiffnecked brethren, the Nephites, who are now hardening their hearts in sin and iniquities, to the knowledge of their Redeemer?
Now these mysteries are not yet fully made known unto me; therefore I shall forbear.
And it may suffice if I only say they are preserved for a wise purpose, which purpose is known unto God; for He doth counsel in wisdom over all His works, and His paths are straight, and His course is one eternal round. (Alma 37:6-12)
Now my son, here is somewhat more I would say unto thee; for I perceive that thy mind is worried concerning the resurrection of the dead.
Behold, I say unto you, that there is no resurrection—or, I would say, in other words, that this mortal does not put on immortality, this corruption does not put on incorruption—until after the coming of Christ.
Behold, He bringeth to pass the resurrection of the dead. But behold, my son, the resurrection is not yet. Now, I unfold unto you a Mystery; nevertheless, there are many Mysteries which are kept, that no one knoweth them save God himself. But I show unto you one thing which I have inquired diligently of God that I might know—that is concerning the Resurrection.
Behold, there is a time appointed that all shall come forth from the dead. Now when this time cometh no one knows, but God knoweth the time which is appointed. (Alma 40:1-4)
Mormon, who abridged the record of Christ’s ministry among the Nephites which was recorded upon the Plates of Nephi caused these words to be written:
And now there cannot be written in this book even a hundreth part of the things which Jesus did truly teach unto this people;
But behold the Plates of Nephi do contain the more part of the things which He taught the people.
And these things have I written, which are a lesser part of the things which He taught the people; and I have written them to the intent that they may be brought again unto this people, from the Gentiles, according to the words which Jesus hath spoken.
And when they shall have received this, which is expedient that they should have first, to try their faith, and if it shall so be that they shall believe these things then shall the greater things be made manifest unto them.
And if it so be that they will not believe these things, then shall the greater things be withheld from them, unto their condemnation.
Behold, I was about to write them, all which were engraven upon the Plates of Nephi, but the Lord forbade it, saying: I will try the faith of My people.
Therefore I, Mormon, do write the things which have been commanded me of the Lord. And now I, Mormon, make an end of my savings, and proceed to write the things which have been commanded me. (III Nephi 26:6-12)
Near the end of the Book of Mormon, Moroni, who abridged the record of the Jaredites, gives in a few words the reason why the Lord should withhold some of the many Mysteries of which He is now the sole possessor:
And the Lord commanded the brother of Jared to go down out of the mount from the presence of the Lord, and write the things which he had seen; and they were forbidden to come unto the children of men until after that he should be lifted up upon the cross; and for this cause did king Mosiah keep them, that they should not come unto the world until after Christ should show himself unto his people.
And after Christ truly had showed himself unto his people he commanded that they should be made manifest.
And now, after that, they have all dwindled in unbelief; and there is none save it be the Lamanites, and they have rejected the gospel of Christ; therefore I am commanded that I should hide them up again in the earth.
Behold, I have written upon these plates the very things which the brother of Jared saw; and there never were greater things made manifest than those which were made manifest unto the brother of Jared.
Wherefore the Lord hath commanded me to write them; and I have written them. And he commanded me that I should seal them up; and he also hath commanded that I should seal up the interpretation thereof; wherefore I have sealed up the interpreters, according to the commandment of the Lord.
For the Lord said unto me: They shall not go forth unto the Gentiles until the day that they shall repent of their iniquity, and become clean before the Lord.
And in that day that they shall exercise faith in me, saith the Lord, even as the brother of Jared did, that they may become sanctified in me, then will I manifest unto them the things which the brother of Jared saw, even to the unfolding unto them all my revelations, saith Jesus Christ, the Son of God, the Father of the heavens and of the earth, and all things that in them are.
And he that will contend against the word of the Lord, let him be accursed; and he that shall deny these things, let him be accursed; for unto them will I show no greater things, saith Jesus Christ; for I am he who speaketh.
And at my command the heavens are opened and are shut; and at my word the earth shall shake; and at my command the inhabitants thereof shall pass away, even so as by fire.
And he that believeth not my words believeth not my disciples; and if it so be that I do not speak, judge ye; for ye shall know that it is I that speaketh, at the last day.
But he that believeth these things which I have spoken, him will I visit with the manifestations of my Spirit, and he shall know and bear record. For because of my Spirit he shall know that these things are true; for it persuadeth men to do good.
And whatsoever thing persuadeth men to do good is of me; for good cometh of none save it be of me. I am the same that leadeth men to all good; he that will not believe my words will not believe me—that I am; and he that will not believe me will not believe the Father who sent me. For behold, I am the Father, I am the light, and the life, and the truth of the world. (Ether 4:1-12)