Unpardonable Sin.

Many seem to have problems with understanding this but if you look at the context carefully all becomes clear.
Jesus uses the phrase when the religious people have accused Him of casting out demons by using the devil. In other words, they see God's Holy Spirit as being the evil one.
I perhaps might take it a step further by saying that this seems to indicate that those who have 'reversed good and evil' are in particularly grave danger of having ignored this most stark of warnings.