First: The sins of commission - Those who walk not in spirit and fulfilling the lust of the flesh.
Galatians 5:19-21 Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, envy, murders, drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God.
Second: The sins of omission - The coward whose works are lukewarm and neither cold nor hot.
Romans 12:9 Let love be without hypocrisy. Abhor what is evil. Cling to what is good.
Finally: The sins committed by unbelievers - like Cain whose offerings are done without faith.
Hebrews 11:4 By faith Abel offered God a better sacrifice than Cain did. By faith he was commended as a righteous man, when God spoke well of his offerings. And by faith he still speaks, even though he is dead.
Hebrews 11:5 By faith Enoch was taken away so that he did not see death, “and was not found, because God had taken him”; for before he was taken he had this testimony, that he pleased God.
Hebrews 11:7 By faith Noah, being divinely warned of things not yet seen, moved with godly fear, prepared an ark for the saving of his household, by which he condemned the world and became heir of the righteousness which is according to faith.
Hebrews 11:8 By faith Abraham obeyed when he was called to go out to the place which he would receive as an inheritance. And he went out, not knowing where he was going.
Hebrews 11:11 By faith Sarah herself also received strength to conceive seed, and she bore a child when she was past the age, because she judged Him faithful who had promised.
And without faith it is impossible to please God, because anyone who comes to him must believe that he exists and that he rewards those who earnestly seek him.
What is then the gravest sin among the three (3)? They are all sins with the same destination - the fiery lake of burning sulfur.
Revelation 21:8 But the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death.