Answer:
Most would believe that there are no unforgetable sins and that GOD forgives them all. There is one mentioned in the Bible, however, called the "blasphemy of the Holy Spirit." This is the ONLY unforgivable sin.
What is it? It is when a person keeps pushing the Holy Spirit away so much that soon enough they are blinded into thinking they are in the right, and in that way, they never repent and is therefore never forgiven. That is the ONLY sin God can never forgive. Mark 3:28-30 is one example of blasphemy against the Spirit an example is that of satan the devil all slander he speaks against God and his people is blasphemy a prime example is found in John chapter 8 where the religious leaders are attributing Jesus works to the devil and saying he has a demon vs 48,52 we also find another example of this in Matt 12:22-32 as well as Luke 12:10 in modern time we have a good example when governments proclaim man made institutions "as the only hope of peace and concord for mankind" that is blasphemy as they cannot bring it about only Gods Kingdom by Christ Jesus will or when certain religious persons claim divinity or political that also is blasphemy hope that helps (pixilated)