Alma 39:6 - 39:10
Alma 39: 6 For behold, if ye deny the Holy Ghost when it once has had place in you, and ye know that ye deny it, behold, this is a sin which is unpardonable; yea, and whosoever murdereth against the light and knowledge of God, it is not easy for him to obtain forgiveness; yea, I say unto you, my son, that it is not easy for him to obtain a forgiveness.
The denying of the Holy Ghost is very serious. But it has to be a denial that is equalivent to knowing that you are saying the Holy Ghost is nothing even when you know in your heart the Holy Ghost has been of great help to you in your life. It would like saying the Sun is not shining when you are looking right at it on a sunny day. Being forgiven of this sin is very difficult if not impossible.
You have to be very, very evil to come into this group. I understand the numbers are very few that quality as Sons of Perdition. Because they have denied the light of the gospel, they will be place in a place that has no light at all.
Alma 39: 7 And now, my son, I would to God that ye had not been guilty of so great a crime. I would not dwell upon your crimes, to harrow up your soul, if it were not for your good.
Alma hopes his son Corianton will never be guilty of such a crime. Alma is bringing up his sins against the Lord to remind him that he has to improve his faithfulness.
Alma 39: 8 But behold, ye cannot hide your crimes from God; and except ye repent they will stand as a testimony against you at the last day.
He is not hiding his sins from God. He knows what Corianton is doing very minute. Unless Corianton repents of his sins, he will have to account for them at the day of judgement.
Alma 39: 9 Now my son, I would that ye should repent and forsake your sins, and go no more after the lusts of your eyes, but cross yourself in all these things; for except ye do this ye can in nowise inherit the kingdom of God. Oh, remember, and take it upon you, and cross yourself in these things.
Alma wants his son to rid himself of his evil habits and his powerful desire to indulge in unrighteous sexual behavior. He needs to increase his regard for the necessity to live according to gospel principles. If he fails to do so, he will not be eligible to enter the kingdom of God
Alma 39: 10 And I command you to take it upon you to counsel with your elder brothers in your undertakings; for behold, thou art in thy youth, and ye stand in need to be nourished by your brothers. And give heed to their counsel.
Alma encourages Corianton to stay close to his older and righteous older brothers. They will help him with his problems and help him stay focused on gospel principles. Talk to them when questions arise and heed their council.