Recently I was asked that question.
Matthew 12:31 “Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.”
Sin is a transgression of God’s Law. If God’s Law were truly done away there would be no sin as Paul confirms. (There would also be no grace, no new covenant and no need for a savior.)
Blasphemy is vilification (especially against God). It comes from a word meaning scurrilous [grossly or obscenely abusive], that is, calumnious [slander] (against man), or (specifically) impious (against God). This comes from two more Greek words that mean a saying, that is to make known, a rumor (“fame”) that is to injure another.
We only worship one God. He has revealed Himself to us in three ways, Father, Son and His Spirit.
I have known some who think the sin is miscegenation. That is to make a mixed race child since it is something that can not be undone. This is a sin, but I do not believe that this is the unforgivable sin. It seems rather plain from the Greek word meanings above.
Blasphemy against the Spirit of God is just what it says. It is obscenely abusive slander. I would recommend one to consider what is obscenely abusive. I would also recommend one to consider that from say, one hundred years ago rather than today.