The unforgivable sin.
There is only one unforgivable sin, and it is to blaspheme, or show contempt to the name of God. This is not committed when you just say “Oh my God” or some similar expression, it means that you totally reject the message of the Gospel – you actually believe that you do not need any “saving.” You therefore deny God and the sacrifice of His Son, the Christ. Like a drug addict or an alcoholic denying their problem, no one can be saved from something they reject.