2 Nephi 9:41-44

41  O then, my beloved brethren, come unto the Lord, the Holy One.  Remember that his paths are righteous.  Behold, the way for man is narrow, but it lieth in a straight course before him, and the keeper of the gate is the Holy One of Israel; and he employeth no servant there; and there is none other way save it be by the gate; for he cannot be deceived, for the Lord God is his name.
42  And whoso knocketh, to him will he open; and the wise, and the learned, and they that are rich, who are puffed up because of their learning, and their wisdom, and their riches—yea, they are they whom he despiseth; and save they shall cast these things away, and consider themselves fools before God, and come down in the depths of humility, he will not open unto them.
43  But the things of the wise and the prudent shall be hid from them forever—yea, that happiness which is prepared for the saints.

2 Nephi 9:41-43 (Emphasis mine)

Jacob calls on his people to remember that the way to "the Holy One of Israel" is on a narrow, but straight course before Him.  When we arrive at the gate, He will be at the gate and "cannot be deceived..."  

This concept of the the narrow but straight path is very familiar to readers of both the Gospels and the Book of Mormon.

13  ¶ Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat:
14  Because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it.

Matthew 7:13-14 (Emphasis mine)

19  And I beheld a rod of iron, and it extended along the bank of the river, and led to the tree by which I stood.
20  And I also beheld a strait and narrow path, which came along by the rod of iron, even to the tree by which I stood; and it also led by the head of the fountain, unto a large and spacious field, as if it had been a world.

1 Nephi 8:19-20 (Emphasis mine)

According to the Phrase Finder, the phrase "strait and narrow" refers to "a conventional and law-abiding course."  (There is also an interesting explanation as to the origin of this phrase as well.)  So, when we're studying the words of Christ, Lehi, and Jacob, we are being told that there is only one way to arrive at the gates of heaven, and that is by following and obeying the teachings and commandments of "the Holy One of Israel."  There is no other way.

Jacob once again reminds us that the "the wise, and the learned, and they that are rich" (v42) will find that the way is not open to them.  As Jacob uses this phrase, we are looking at those that will not humble themselves before God.  They are under the impression that they are above all this.  They will refuse to humble themselves.  As a consequence, they will find that gate will not be opened for them.  The happiness that is "...prepared for the saints..." (v43) will not be made available to to them.  

Paul wrote:

14  But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.

1 Corinthians 2:14

Only through "...the Spirit of God..." can we find that, when we arrive at the gate, the Gatekeeper will open it for us.

44  O, my beloved brethren, remember my words.  Behold, I take off my garments, and I shake them before you; I pray the God of my salvation that he view me with his all–searching eye; wherefore, ye shall know at the last day, when all men shall be judged of their works, that the God of Israel did witness that I shook your iniquities from my soul, and that I stand with brightness before him, and am rid of your blood.

2 Nephi 9:44 (Emphasis mine)

Jacob wants his listeners (as well as readers) to know and clearly understand that they have been warned.  They have had the word of the Lord preached to them.  They have learned about what awaits them in the eternities.  Some highlights of his words so far:

• Because of the fall, "...there must needs be a power of resurrection..." (v6) or death would have a permanent claim on man.
• Because of the fall, there "...must needs be an infinite atonement..." (v7)
• Without the atonement and resurrection, "...our spirits must have become alike unto him, and we become devils, angels to a devil, to be shut out from the presence of our God..." (v9)
• "...[B]ecause of the way of deliverance of our God, the Holy One of Israel, this death, of which I have spoken, which is the temporal, shall deliver up its dead; which death is the grave." (v11) 
• "...[T]he paradise of God must deliver up the spirits of the righteous, and the grave deliver up the body of the righteous; and the spirit and the body is restored to itself again, and all men become incorruptible, and immortal, and they are living souls, having a perfect knowledge like unto us in the flesh, save it be that our knowledge shall be perfect." (v13) 
• "...[T]hey who are filthy shall be filthy still; wherefore, they who are filthy are the devil and his angels; and they shall go away into everlasting fire, prepared for them; and their torment is as a lake of fire and brimstone, whose flame ascendeth up forever and ever and has no end. (v16) 
• "...[T]o be learned is good if they hearken unto the counsels of God. (v29)

It is clear from this quick overview of Jacob's teachings that he was very clear about the consequences of obeying and not obeying the word of the Lord.  No one hearing or reading his words would be able to say we didn't know.

In the April 2010 conference, Elder David A. Bednar said:

"Early warning signals are evident in many aspects of our lives. For example, a fever can be a first symptom of sickness or disease. Various financial and labor market indicators are used to forecast future trends in local and national economies. And depending upon the area of the world in which we live, we may receive flood, avalanche, hurricane, tsunami, tornado, or winter storm warnings.  We also are blessed by spiritual early warning signals as a source of protection and direction in our lives. Recall how Noah was alerted by God of things not yet seen, and he “prepared [the] ark to the saving of his house” (Hebrews 11:7)...Spiritual warnings should lead to increasingly vigilant watching. You and I live in “a day of warning” (D&C 63:58). And because we have been and will be warned, we need to be, as the Apostle Paul admonished, “watching … with all perseverance” (Ephesians 6:18)." (Emphasis mine)

In the October 1999 General Conference, Elder David R. Stone of the Second Quorum of the Seventy said:

"But we also have our spiritual hurricane guardians, those whose calling it is to watch and warn, helping us avoid spiritual damage, destruction, and even death. Our watchmen on the tower are known to us as apostles and prophets. They are our spiritual eyes in the sky, and they know, through inspiration and insight and pure intelligence, the course these storms may take. They continue to raise their voices in warning to tell us of the tragic consequences of willful and wanton violations of the Lord’s commandments. To intentionally ignore their warnings is to court misery, sorrow, and ruin. To follow them is to follow the chosen servants of the Lord into spiritual pastures of peace and plenty ... There is peace and tranquility, there is solace and safety in His gospel. If we will but listen to those whose calling it is to watch and warn, if we will give heed to the words of the Master Himself, then our spiritual house will stand firm, and we can let the rain descend and the floods come and the winds blow and beat upon our house, because we are founded upon that rock (see Matt. 7:24–25)." (Emphasis mine)

May we constantly remember the warnings we get from those called, as was Jacob, to ensure  we know and understand the words of the Lord.