Question: What is this sin which is unpardonable? or the blasphemy against the Holy Spirit, what is it?
Response: Blasphemy against the Holy Spirit and the unpardonable sin are the same thing: at their core they are rejection of Jesus Christ (i.e., disbelief). Jesus' reference to the "blasphemy against the Spirit" comes in the context of His detractors claiming that He did what He did by the power of the devil (read Matt.12:22-32 and note Mk.3:30: "He said this because they were saying 'He has an evil spirit'") - this "blasphemes" the Holy Spirit (who actually empowered our Lord's miracles) by falsely claiming that Jesus was not doing God's will and was not of God; at the same time it is a rejection of Jesus Himself (rejecting His work necessarily rejects Him too) so that those who say that Jesus didn't really do God's work by God's power are really rejecting Jesus and consequently will die in their sins for their lack of faith in Him (i.e., they are unbelievers). Rejecting God's solution to the problem of sin, Jesus Christ and His death on the cross for us, is unforgivable by definition, for it is unbelief - that is the unpardonable sin. The one thing we can't do and still be saved is to turn our back on the Savior of the world, the Lord Jesus Christ.
Here is a trustworthy saying: If we died with Him, we will also live with Him; if we persevere, we will also reign with Him. If we disown Him, He will also disown us; If we are faithless, He will remain faithful, for He cannot disown Himself.
2nd Timothy 2:11-13
Please see also the links:
Have I committed the unforgivable sin?
Have I blasphemed the Holy Spirit?
Blasphemy against the Holy Spirit
Rejecting Christ is blaspheming the Spirit
And you can find out more about this and important related topics at the following link:
Bible Basics 3B: Hamartiology: The Biblical Study of Sin.
In His name,