What do you think the greatest sin is according to the Bible?
"He has shown you, O man, what is good and what the Lord requires of you. But to do justly..and to love mercy...and to walk humbly with your God."
Basically, the truly egregious sins are the ones that others choose to commit. All of my transgressions are relatively minor. (And not even my fault, to be honest.)
It appears that Jesus taught that it is "blasphemy against the Holy Ghost" (Matthew 12:31 KJV), which is unforgivable.
“Can we conceive of a greater incongruity, than for a Christian to go from his closet, where he has been praying for his enemies, and command his troops to plunge the weapons of death with fiend like fury, into the hearts of those very enemies? In the one case, he happily resembles his dying Master; but whom does he resemble in the other? Jesus prayed for his murderers. Christians murder those for whom they pray.” - Henry Grew, 1828
The hound of Jewish monotheism.
"For us there is one God. He is the Father." (I Corinthians 8:6)
Well to make it short and sweet Biblestudent laid it out precisely, the only sin that is unforgivable is blasphemy of the Holy Spirit (which to put in other terms every other sin is forgivable). That is because by blaspheming the Holy Spirit we cut off the only help we have.
Matt that means even idolatry is forgivable under the right conditions. (it may be difficult but not impossible.)
St. Peter (03-18-2011)
The cause of all sin is unbelief towards God.
If you don't believe God, then you don't fear God, and if you don't fear God, you will please yourself, and if it pleases you to blaspheme, or murder, or steal, or hate your neighbor you will do as you please without fear.
If you don't believe God then you can't love God, and if you don't love God you will love yourself.
The fear of the Lord is the beginning of knowledge.
And, we love God because he first loved us.
But if you don't believe God you can't respond to God's love or chastisements.