Free Will
God will never force the will.
Christ is ready to set us free from sin, but He does not force the will; and if by persistent transgression the will itself is wholly bent on evil, and we do not desire to be set free, if we will not accept His grace, what more can He do? We have destroyed ourselves by our determined rejection of His love. "Behold, now is the accepted time; behold, now is the day of salvation." "Today if ye will hear His voice, harden not your hearts." 2 Corinthians 6:2; Hebrews 3:7, 8.
- Ellen White, Steps to Christ, chapter 3
It is the Holy Spirit that convicts us of sin.
And when he [the Holy Spirit] is come, he will reprove the world of sin, and of righteousness, and of judgment: Of sin, because they believe not on me; Of righteousness, because I go to my Father, and ye see me no more; Of judgment, because the prince of this world is judged.
- John 16:8-11 (KJV)
When we are convicted of a sin, we have a choice: You either say "Yes, you're right" and repent (obedience), or you will say something like "No, I know better" and do whatever you like (rebellion).
Unfortunately what happens is that, as time goes on, it becomes easier and easier to sin to the point where now there is no forgiveness anymore. You are now so "out of sync" with how things work in heaven, that it has become impossible for you to "fit in". At some level you have decided that God's Law is "not for you", and that you rather join satan's rebellion.
There just comes a point where there is no more forgiveness.
Most Christians assume that if they go to church once a week for an hour that they are saved, but that's not necessarily the case.
Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
- Matthew 7:21-23 (KJV)
This is how you will be judged:
When the Son of man shall come in his glory, and all the holy angels with him, then shall he sit upon the throne of his glory: And before him shall be gathered all nations: and he shall separate them one from another, as a shepherd divideth his sheep from the goats: And he shall set the sheep on his right hand, but the goats on the left. Then shall the King say unto them on his right hand, Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world: For I was an hungred, and ye gave me meat: I was thirsty, and ye gave me drink: I was a stranger, and ye took me in: Naked, and ye clothed me: I was sick, and ye visited me: I was in prison, and ye came unto me. Then shall the righteous answer him, saying, Lord, when saw we thee an hungred, and fed thee? or thirsty, and gave thee drink? When saw we thee a stranger, and took thee in? or naked, and clothed thee? Or when saw we thee sick, or in prison, and came unto thee? And the King shall answer and say unto them, Verily I say unto you, Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me. Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels: For I was an hungred, and ye gave me no meat: I was thirsty, and ye gave me no drink: I was a stranger, and ye took me not in: naked, and ye clothed me not: sick, and in prison, and ye visited me not. Then shall they also answer him, saying, Lord, when saw we thee an hungred, or athirst, or a stranger, or naked, or sick, or in prison, and did not minister unto thee? Then shall he answer them, saying, Verily I say unto you, Inasmuch as ye did it not to one of the least of these, ye did it not to me. And these shall go away into everlasting punishment: but the righteous into life eternal.
- Matthew 25:31-46 (KJV)
If you follow the promptings of the Holy Spirit, you will do all those things automatically, and you are in compliance with God's Law.
But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, meekness, temperance: against such there is no law.
- Galatians 5:22-23 (KJV)
Obedience to God's Law will change you in a good way, and will prepare you for heaven.
That's why you receive the Holy Spirit as a deposit first, so that God can see where your heart really lies.
I the Lord search the heart, I try the reins, even to give every man according to his ways, and according to the fruit of his doings.
- Jeremiah 17:10 (KJV)
Once you receive the Holy Spirit without measure (in heaven), the conviction of sin will be so strong, that you don't even want to sin anymore. No more sin, imagine that. 
All you need to is ask.
How much more shall your heavenly Father give the Holy Spirit to them that ask him?
- Luke 11:13 (KJV)