There are many important things to know about the Holy Ghost that are beyond the scope of this book.
For the purposes of the topic of repentance, the key point to understand is that the Holy Ghost is an information channel through which God teaches people more about himself, themselves, and every other aspect of reality:
[God] manifesteth himself unto all those who believe in him, by the power of the Holy Ghost... (2 Nephi 26:13)
What God can teach you through the Holy Ghost cannot be compared with what can be received by a person in their default state.
God is available to all people through the Holy Ghost, but not all people qualify themselves equally for this privilege.
The natural state of man is contrary to the Holy Ghost:
For the natural man is an enemy to God, and has been from the fall of Adam, and will be, forever and ever, unless he yields to the enticings of the Holy Spirit, and putteth off the natural man.... (Mosiah 3:19)
As a person rebels against the increment of conscience they have naturally, they disqualify themselves from additional information through the Holy Ghost.
What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own? (1 Corinthians 6:19)
I say unto you, that the man that doeth this, the same cometh out in open rebellion against God; therefore he listeth to obey the evil spirit, and becometh an enemy to all righteousness; therefore, the Lord has no place in him, for he dwelleth not in unholy temples. (Mosiah 2:37)
And this I know, because the Lord hath said he dwelleth not in unholy temples, but in the hearts of the righteous doth he dwell.... (Alma 34:36)
Whenever people do less than their best, they attenuate the information flow through the Holy Ghost towards the minimal increment available to all people.
Repentance and baptism leverage the sacrifice of Jesus Christ to restore the connection between an individual and God through the Holy Ghost through forgiveness of past sins. When this happens, the moment it happens is called the baptism of fire, because the person is suddenly exposed to a significantly increased increment of God’s intelligence, purging away everything that can only exist in the darkness of one’s previous ignorance:
And it came to pass when they were all baptized and had come up out of the water, the Holy Ghost did fall upon them, and they were filled with the Holy Ghost and with fire. (3 Nephi 19:13)
This connection is required in order for a person to learn sufficiently about God to gain greater joy through becoming like him.