Luke 8:10
And he said, Unto you it is given to know the mysteries of the kingdom of God: but to others in parables; that seeing they might not see, and hearing they might not understand
1 Nephi 1:1
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.
1 Nephi 10:19
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.
Alma 26:22
Yea, he that repenteth and exerciseth faith, and bringeth forth good works, and prayeth continually without ceasing—unto such it is given to know the mysteries of God; yea, unto such it shall be given to reveal things which never have been revealed; yea, and it shall be given unto such to bring thousands of souls to repentance, even as it has been given unto us to bring these our brethren to repentance.
D&C 84:19
And this greater priesthood administereth the gospel and holdeth the key of the mysteries of the kingdom, even the key of the knowledge of God.
D&C 10:64
Therefore, I will unfold unto them this great mystery
I believe the mysteries are the things that are made mention of but often like Paul says that there were unspeakable things that could not be uttered. I believe the mysteries are the things we week for and dive deeper into asking God for testimony and personal revelation to further understand. I believe our Heavenly Father delights in teaching us more as we are able to bear it.
“God hath not revealed anything to Joseph, but what he will make known unto the Twelve, and even the least Saint may know all things as fast as he is able to bear them.” (Teachings of the Prophet Joseph Smith, pg. 149)
The Gospel isn't an exclusive club privileged for a few, but anyone who will be obedient can obtain all the promises in the scriptures.
For deeper dives on this topic here's a great article
https://scripturenotes.com/are-we-seeking-the-mysteries-of-god