In Alma 12:9 it says, "It is given unto many to know the mysteries." How can they be mysteries if they are known to many? They are things which are kept sacred, and there is a limit to who learns of them, namely the covenant people. The people who are obedient, who with their faith and diligence according to their heed, allow it to be revealed to them, and they are shown more incrementally.
In addition, people may be given many opportunities, but not all absorb them to the same degree. Alma was teaching people who did not accept those mysteries. He was essentially saying to them, "We as Nephites do not have anything that is exclusively private. I am willing to give the mysteries of God to all of you. It is given to many, and you can absorb this at whatever speed or depth you are able." It was a very kind position for him take under the circumstances.
But, as he explained, God will "impart only according to the portion of his word which he doth grant unto the children of men, according to the heed and diligence which they give unto him" (12:9), and that statement is, in all of scripture, the best formula for how to understand and appreciate what are called here "the mysteries."
Heed and diligence. What do those words mean? To heed means hearing and paying close attention. The words are "according to the heed." In other words, one can heed a little bit or a lot. One can heed certain things, while wrongly overlooking to heed other things. The people of Ammonihah were not heedful at all. They were very dismissive, very proud, not teachable, and not willing to listen.
In 1 Nephi 16:28, Nephi said that the Liahona "worked according to the faith and diligence and heed which thou didst give unto them." In that scripture, faith, diligence, and heed applied only to the use of the Liahona, whereas Alma now took that sacred operation which the prophets used to make that instrument work and applied it to people’s spiritual lives.
Our leaders continually tell us to heed the scriptures, particularly the Book of Mormon. One should not just read it through, but also really read it and understand. Sometimes that takes somebody to teach us and others to discuss it with. It is assumed that obedience will be part of that learning process. The words heed, hearken, and obey are linguistically very closely related. We should evaluate how well we are heeding today in comparison with how well we heeded in previous years.
We also need to be diligent to obtain the mysteries of God, and in order to be diligent we must work. Alma addressed the mysteries of God being unfolded. King Benjamin unfolded mysteries. By that, he meant "sacred revelations." Often these mysteries, or sacred revelations, have a great deal to do with the temple; they are not mysterious in the modern sense of the word. If the word sacred revelation is used whenever the word mystery appears—and that is what King Benjamin revealed, namely "the words that the angel spoke to me"—the text becomes much clearer. There is nothing really "mysterious" about it. It is sacred revelation. The temple is a place where many of the sacred revelations, especially the foreordained plan of God, how we will progress, and why we are here, are all made manifest.
In Amos 3, it is written, "God will do nothing save he reveal his mysteries unto his servants the prophets." The word mystery in that context refers to the prophetic experience of being brought up into the Council, amidst the hosts of Heaven, where the will of God and the judgments of God are manifest. The word plan appears seven times in chapter 12. This chapter has the highest density of references to the plan of salvation in any chapter in the Book of Mormon. It is the plan that is revealed to the prophet, so people know what the program is. The plan is really the greatest mystery, because it is the decree of the Council of Heaven, which is what the word mystery means in this context.
It is interesting that the word mystery is used in the very first verse of the Book of Mormon. Nephi mentions that he had a great knowledge of the goodness and the mysteries of God (1 Nephi 1:1), and he gives that as the reason for making his sacred record. Some people believe that they will never find out what the mysteries are until they arrive in Heaven, but the prophets reveal mysteries frequently.
Book of Mormon Central, "Why Does Alma Mention ‘the Plan’ Ten Times in His Words to Corianton? (Alma 42:13)," KnoWhy 150 (July 25, 2016).
Book of Mormon Central, "Why Did Alma Teach His Opponents about the Temple?" KnoWhy 119 (June 10, 2016).