What is the Unforgivable Sin?

The scriptures tell us that there is only one unforgivable sin. Since this sin is so serious--a sin from which one can never recover their standing with God--it is critical to look at what the scriptures say, to gain a better understanding of just what this particular sin entails.

Some groups, by their continuing to shun someone long after the person is no longer sinning or because the person holds different beliefs, are actually treating that person as if he/she had committed the unforgivable sin. Those doing the shunning are engaging in a practice of unChristian non-forgiveness.
 
What the Scriptures Say About the Unforgivable Sin

Matthew 12:31 reads:  "And so I tell you, every kind of sin and slander can be forgiven, but blasphemy against the Spirit will not be forgiven."

Important Questions
Question 1: What is blasphemy?
Question 2: What type of blasphemy?
Question 3: What was the Spirit?
Question 4: Why so serious?

What is Blasphemy?
The dictionary definition of blasphemy tells us that there is a greater application than merely "speaking against." It means: the act of insulting or showing contempt or lack of reverence for God. It is irreverence toward something considered sacred or inviolable. And certainly, if we look at the context in which the verse above was uttered, we can determine that this was the type of blasphemy meant.

What Type of Blasphemy?
It clearly wasn't run-of-the-mill blasphemy, as we sometimes think of it today, i.e. when someone mocks, blasphemes or runs down another. The verse said that every type of sin would be forgiven (except one) and that included slander.

Context (Matthew 12:22-37)
In that same chapter, in verse 22, Jesus healed a demon-possessed man. Many marveled at this miraculous act. But... how did the Pharisees react? Did they recognize that Jesus had healed the man through divine agency?

Demonic Source Versus a Divine Source
In verse 24, note that the Pharisees attributed a demonic source to workings of Spirit. They said: "It is only by Beelzebul, the prince of the demons, that this fellow drives out the demons."

So they dismissed that this power was divine and they blasphemed by attributing a demonic source to workings of spirit.

What Was the Spirit?
Jesus answered them and he said in verse 28: "But if it is by the Spirit of God that I drive out demons, then the kingdom of God has come to you."

So we learn something important about what the "spirit" was. Jesus identified that it was the Spirit of God. 

Why so Serious? 
Jesus reiterated in verse 32: "Anyone who speaks a word against the Son of Man will be forgiven (run-of the mill blasphemy) but anyone who speaks against (dismissing/attributing a demonic source to) the Holy Spirit (religious blasphemy) will not be forgiven, either in this age or in the age to come."

So we learn that the type of blasphemy that was and is so serious is speaking against the Holy Spirit and even more: attributing a demonic source to workings of Holy Spirit.

And later, in verse 37, "For by your words you will be acquitted, and by your words you will be condemned.”

Good Versus Evil
Those who are condemned forever more are those who attribute a demonic source to miracles or other things that are powered, as it were, by means of the Holy Spirit or Spirit of God.

Very serious indeed because it would be akin to saying that what was coming from God, was in fact evil and coming from the Devil.

What Has Been Learned About the Unforgivable Sin?
So far, we have learned, in context, that the Pharisees blasphemed in a religious sense, they denied the workings of the spirit of God and said Jesus was using demonic power to do what he did. 

Conclusion & Takeaway
Blasphemy against the Spirit is attributing a demonic source to workings of Holy Spirit.