2 Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God:
3 And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
4 Ye are of God, little children, and have overcome them: because greater is he that is in you, than he that is in the world.
5 They are of the world: therefore speak they of the world, and the world heareth them.
6 We are of God: he that knoweth God heareth us; he that is not of God heareth not us. Hereby know we the spirit of truth, and the spirit of error. (1 John 4:1-6)
There are two types of people in this world when it comes to accepting truth: There are the children of the devil, who are not interested in accepting new truth. There are those who will accept some truth. The first category has many more members than the last.
Truth is progressive, not finite. It is not possible to know all truth. Truth is only limited by what portion of it pertains to a certain glory. The truth pertaining to the telestial glory, for example, is finite, as well as the truth pertaining to the terrestrial glory, as well as truth pertaining to the celestial glory. There is truth and glories above that of which we are not aware, and cannot be aware while in the telestial kingdom. Consider the following:
For any two people, one will have more truth than the other. However, this is more complicated than one person knowing more about every topic than the other, because of the multi-dimensional nature of truth. The Father and Son have more truth than anyone who has ever or will ever live in the telestial world.
There are three types of people in this world when it comes to glory had before the world was: There are those who came from and are inheriting a telestial glory, those who came from a terrestrial but are condescending to a telestial glory, and those who came from a celestial but are condescending to a telestial glory. The first category has many more members here than the second, and the second many more than the last.
Graciously, God veils a person's knowledge from before this world when they are born. The main reason this is so beneficial is that it gives all people access to higher intelligences than their own. This may be puzzling at first, because God veils a person's knowledge when they are born. What he does not do, and cannot do, is remove their desire for further light and truth. Many mistakenly believe that Jesus was born with some divine ability to be more righteous than everyone else. In fact, he was just as human as anyone else. There was nothing about his body that made it any easier to do God's will. What was different was that he had a stronger desire to seek light and truth than anyone who has ever lived. This was not a supernatural gift, it was an unavoidable consequence of the refinement of his soul that had occurred in times before this creation. Likewise, any person born here who possessed a higher degree of glory before this world was retains the desire for truth developed previously in spite of lacking the knowledge they had then.
In this graphic, you see persons a, b, and c (time is the x-axis). All of them start at the same amount of light. Light and truth is offered to all people equally. When one acquires more, it is through greater effort in searching and more consistent obedience. At first, person b acquires light and truth at a much faster rate than his fellows. However, he wanes in effort and therefore stops progressing. Person a overtakes person b, and in the end will acquire much more light and truth than person b. Person c is inconsistent in their approach, and even goes through a period of disobedience where they lose light and truth, before regaining some through sore repentance.
You'll notice on this graph the presence of a horizontal range. The dot represents the current light and truth (a 1 dimensional summary of the orange plane above for simplicity). Above this dot lies the new truth that this person does not yet have but is capable of receiving. Below this dot is the range of truth that this person is capable of teaching.
In this graphic, person a can teach person b, and person b can teach person c, but person a cannot teach person c. If they tried, person c would reject person a's teachings and subsequently have light and truth taken away. They would lose enough light that they would try to kill person a.
The spirit of the antichrist is to turn away from higher light and truth. It is a pitfall that every person born on this earth can commit. To say that differently, there exists truth for every person that is so advanced as to be beyond their capacity to see its truthfulness. If such a truth is presented to you, you will turn away from it, lose truth, and seek to kill the person who taught it to you.
We can extend our capacity to accept new light and truth through penitent prayer and humility towards God. When we truly believe we know nothing, we are much more willing to accept new truth. When we are totally willing to let go of anything we think we know, we are much more willing to accept new truth. When we refuse to dismiss anything out of hand (as John instructs us--"try the spirits" or test new truth to see its fruits), we are much more willing to accept new truth. When we refuse to "set up stakes for God," or dictate to him what is right and what is wrong, we are much more willing to accept new truth. When we realize that what was right before and what God might have inspired us to accept before could be superseded by something higher (as happened to Nephi when God taught him a higher commandment regarding killing), we are much more willing to accept new truth. It is possible to come to the point where no truth pertaining to the Celestial, Terrestrial, or Telestial kingdom has power to derail you, despite the fact that you do not yet know them. This is when your calling and election is made sure, or rather this is what it means and what it takes.
Now, you might wonder, couldn't person a just choose to teach things they know person c is capable of receiving? The problem here is that person a has an obligation--a covenant--to stand up for the truth at all times and in all places. They may very well be able to speak in enough parables, or in enough vague generalities--as Jesus did--to avoid someone trying to kill them for a time, but eventually they will always come up against situations where they only thing that can be said to challenge falsehood lies outside the range of the audience's truth. This is how Jesus and many others came to die.
Jesus was able to teach the general public in his mortal ministry because he, like us all, was born at the bottom of the light and truth continuum. Through his unfailing heed and perfect obedience, he advanced to record levels of light and truth. At the time of his 3 year ministry, he was already so far ahead of his fellows that from the beginning men sought to kill him. Meanwhile, when he appeared to the Nephites in the Americas after his resurrection, he taught those same things and higher truths that those in Jerusalem were not prepared to hear. No one sought to kill him. This is because everyone below the necessary threshold was killed prior to his appearance.
Those who seek after God with all their heart, might, mind, and strength will only be of use to those of lower levels of glory while there exists an overlap in the truth the one persons possesses and the capabilities of the other.