This is the unpardonable sin. It says if a Christian commits blasphemy against the Holy Spirit, there is no forgiveness for it. What does He mean? He is pronouncing the terrible fate of those who choose the wrong side. If we blaspheme the Spirit of God, which is God's power and the agency by which He works, we have determined that God has no power. This is, of course, a lie, because, as He just explained in the Parable of the Strong Man (Matthew 12:29), God is the most powerful Being.
If we deny God's power, it is denying God Himself. He says a person can blaspheme the Son of God, but try blaspheming God's power! Doing so makes Him into something else—it changes His nature (Romans 1:22-23)—and that is unforgivable. So we must choose our side carefully, because if we blaspheme what God is able to do, guess where we end up? In the Lake of Fire.
There is more to the unpardonable sin, but this is part of it.
Richard T. Ritenbaugh
Parables of Matthew 13 (Part 2): Leaven
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