To sin is to do, or even think, something contrary to the will of God. In the Old Testament the Israelites were given the Ten Commandments and many other specific laws to govern their life. In the New Testament the "spirit" of the Law rather than the "letter" of the Law was emphasized. Jesus said, " 'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment. And the second is like it: 'Love your neighbor as yourself.' All the Law and the Prophets hang on these two commandments." (Matt. 22:37-40)
Concerning any difference in types of sin, James writes in his epistle: "For whoever keeps the whole law and yet stumbles at just one point is guilty of breaking all of it." (James 2:10, italics added) The Apostle Paul seems to single out fornication as an especially serious sin, saying, "All other sins a man commits are outside his body, but he who sins sexually sins against his own body." (I Cor. 6:18) The Corinthian brethren had a special problem with this kind of sin, and Paul needed to teach them how important it was to keep themselves pure, since our "body is a temple of the Holy Spirit." (verse 19) Jesus and the apostles warned against hypocrisy, rage and other sinful behavior. James tells us that even showing favoritism is a serious sin. (James 2:1-9)
We know that "death came to all men, because all have sinned." (Rom. 5:12) And we know that the "wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord." (Rom. 6:23) Our only hope for life after death is the Ransom paid by Jesus (I Tim. 2:5,6) Likewise, our only hope for a continued relationship with our Heavenly Father is also through faith in Jesus. "If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness." (I John 1:9)
In the Old Testament there was a difference in dealing with sins of "ignorance" (or "negligence" or "frailty") as opposed to "presumptuous" (or "deliberate, unrepentant") sin, as discussed in Smith's Bible Dictionary under the topic of the "Sin Offering." We read in Numbers 15:27 that if a person sinned unintentionally, he would bring a year-old female goat for a sin offering. The priest would make atonement, and the person would be forgiven. However, Numbers 15:30 reads, "But anyone who sins defiantly… blasphemes the LORD, and that person must be cut off from his people." Soon after this command was given to the people, a man was found gathering wood on the Sabbath and was brought to Moses and Aaron, because "it was not clear what should be done to him. Then the LORD said to Moses, 'The man must die.' " (Num. 15:32-35) This might seem very harsh to us. The people who caught the man breaking the Sabbath could not tell what to do with him, but God who reads the heart could no doubt see that the man's sin was defiant and therefore pronounced his sentence.
In the same way now, while we are promised forgiveness for the sins we are truly sorry for, we must guard our attitude. The Apostle John warned that there is a "sin that leads to death." (I John 5:16) Jesus warned in Matthew 12:31-32 about the sin against the Holy Spirit, which will not be forgiven. This is also sometimes called the sin against light or knowledge. Paul describes this sin very well in Hebrews 10:26: "If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left." (See also Heb. 6:4-6)
From this we see that the greatest sin is one that is committed by a person who has accepted the Lord Jesus as his/her Savior, promised to follow the Lord unto death, received the Holy Spirit, and then through pride and arrogance decides he/she doesn't need a Savior and turns to a life of defiant sin. There would be no resurrection for that person.