What is the greatest sin?

Hmm . . .. The "greatest sin"? I think you mean the worst sin, yes? (This is what is called good-natured ribbing!)

Seriously, though, according to Jesus, the worst sin is the refusal to believe in the One whom God has sent: Jesus Himself (also known as the Christ, the Son of the Living God). All other sins pale in comparison to the sin of obdurate, unrepentant unbelief.

"This is the work of God, that you believe in Him whom He has sent" (Jn 6:28).

Also,

"Whoever believes in [Jesus] is not condemned, but whoever does not believe stands condemned already because he has not believed in the name of God's one and only Son" (Jn 3:18).

In other words,

unbelief=condemnation

When I think about it, there's nothing worse than being condemned, and thus separated from God forever.

Furthermore, as James reminds us,

"For whoever keeps the whole law and yet stumbles in one point, he has become guilty of all" (2:10).

It's not a matter of which sins one has committed, or how bad or not-so-bad the sins are, but it's a matter of belief or unbelief. As offensive as this concept is to people living in an era of political correctness, a generation in which closed-mindedness parades as open-mindedness, and tolerance of any and all beliefs asserts itself as the ultimate virtue, it's refreshing--to me at least--to come back to the simple, profound, and uncompromising truth that the sin of unbelief is what separates the saints from the aints.

The greatest sin is the one where a person was separated from a loving God by their own personal action or thought and would require the sacrifice of the Lord and Saviour, Jesus Christ, to redeem them from their terrible state.

Now if it is our desire to judge, through a seemly harmless discussion, if one sin is greater than another, aren’t we taking upon ourselves the idea that we are qualified to do so? Even if we examine scripture, aren’t we are still trying to develop a measuring stick so we can judge sin?

Every person is or has been separated from God making every person guilty of committing the greatest sin (Romans 1-3 (NET)). Every sin committed after that, can do no worse.

The greatest sin as defined in the Book of Acts is to blaspheme the Holy Spirit. To blaspheme or lie to the Holy Spirit is to deny the power it beholds as the third person of the trinity. To blaspheme the Holy Spirit is to blaspheme God as they are both part of the whole triune God.

Also mentioned in the Gospels:

31 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.

32 And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come. (Matthew 12 KJV)

The greatest sin a man can commit is DISOBEDIENCE to God. The act of disobedience makes you break the rules of God. The sole aim of Satan is to have man disobey God like he did in the garden of Eden.

1 Samuel 15:22-23

22 And Samuel said, has the LORD has great delight in burnt offerings and sacrifices, as in obeying the voice of the LORD? Behold, to obey is better than sacrifice, and to hearken than the fat of rams

23 For rebellion is as the sin of witchcraft, and stubbornness is as iniquity and idolatry. Because thou hast rejected the word of the LORD, he hath also rejected thee from being king

In the Ten Commandments, God said you should not do this and you should not do that. There are other places in the Bible where God said we should not do other things apart from the ones mentioned in the Exodus 20. The life of Israelites/Christian is based on following God's detail instructions. In the passage mentioned above, Saul was the victim.

In this new age, our daily living and place in christ is centered around hearing from him and following his lead

Ephesians 2:1-2
1 As for you, you were dead in your transgressions and sins,
2 in which you used to live when you followed the ways of this world and of the ruler of the kingdom of the air, the spirit who is now at work in those who are disobedient

This is God's commandment to everybody

Mark 9:7 And there was a cloud that overshadowed them: and a voice came out of the cloud, saying, This is my beloved Son: hear him

Jesus also said in John 10:27

My sheep hear my voice, and I know them, and they follow me

To follow somebody's lead you have to obey them

Romans 8:14 For as many as are led by the Spirit of God, they are the sons of God

Simply disobeying God makes it the greatest sin of all time. It was the first sin, committed by the first man. It was what took them out of the garden. It was what took Satan out of heaven.

The greatest commandment is given to us by Jesus.

Matthew 22:36-40 NIV
36 “Teacher, which is the greatest commandment in the Law?”

37 Jesus replied: “‘Love the Lord your God with all your heart and with all your soul and with all your mind.’[a] 38 This is the first and greatest commandment. 39 And the second is like it: ‘Love your neighbor as yourself.’[b] 40 All the Law and the Prophets hang on these two commandments.”

I believe it'd follow that the greatest sin would be breaking this commandment. So I would say acting in an unloving manner would be the greatest sin. Being unloving towards God would be the greatest, followed by being unloving towards your neighbor.

Also, since the commandment is the action of loving, having a lack of love could also be considered a sin.

Putting sin in that light is convicting.