No sin is greater than another sin in the eternal sense. All sin separates us from God, and all sin needs to be atoned for. Also, there is no “greatest sin” in the sense of “mortal” and “venial” sins, as the Catholic Church teaches. All sins are “mortal” sins in that even one sin makes the offender worthy of spiritual death and eternal separation from God. At the same time, the Bible does state that on the day of judgment some sins will merit greater punishment than others (Matthew 11:22, 24; Luke 10:12, 14).
Jesus also referred to one sin being a “greater” sin (although not the “greatest”) in John 19:11. Speaking to Pontius Pilate, He said that the one who had handed Him over to Pilate was guilty of the “greater sin.” He meant that the guilt of the person who delivered Him to Pilate, whether Judas or Caiaphas, was greater than Pilate’s because of the deliberate and cold act of handing Jesus over after seeing the overwhelming evidence of His miracles and teaching, all pointing unmistakably to Him as the Messiah and the Son of God. That sin was greater than that of those who were ignorant of Him. This could indicate that those who have been given knowledge of Jesus as the Son of God and still reject Him would be subject to a greater punishment than those who remain ignorant of Him: "If you were blind, you would not be guilty of sin; but now that you claim you can see, your guilt remains” (John 9:41).
These incidents, however, do not prove that one sin is the “greatest sin” of all. Proverbs 6:16–19 is a catalog of the seven sins God hates and are detestable to Him: “Haughty eyes, a lying tongue, hands that shed innocent blood, a heart that devises wicked schemes, feet that are quick to rush into evil, a false witness who pours out lies and a man who stirs up dissension among brothers.” But none of the seven are identified as a greater sin than any of the others, and none are identified as the greatest sin.
Although the Bible doesn’t name any one sin as the greatest sin, it does refer to the unpardonable sin, which is the sin of unbelief. There is no pardon for a person who dies in unbelief. The Bible is clear that, in His love for mankind, God provided the means of eternal salvation—Jesus Christ and His death on the cross—for “whoever believes in Him” (John 3:16). The only condition under which forgiveness would not be granted concerns those who reject the only means of salvation. Jesus said, “I am the way and the truth and the life. No one comes to the Father except through me” (John 14:6), making it clear that He and He alone is the path to God and salvation. To reject the only means of salvation is unpardonable and, in that sense, is the greatest sin of all.