According to Matthew 12:32, "whoever speaks a word against the Son of Man, it shall be forgiven him; but whoever speaks against the Holy Spirit, it shall not be forgiven him, either in this age or in the age to come." ( NASB)
"[B]lasphemy of the Holy Spirit is saying that Jesus did His miracles by the power of the devil."^^ Others theologians believe that blasphemy of the Holy Spirit refers to final unrepentance. Some of these see a parallel in Hebrews 6:4-6 and 1 John 5:16; _See Matthew 12:22-32_
-  (MP3), The Unforgivable Sin
- What is blasphemy of the Holy Spirit? Can a Christian commit it? (YouTube video) (CARM)
Why is it unforgivable?
"This is unforgivable because it struck at the very heart of the redemptive work of God in Christ. It struck at the very nature of Jesus’ ministry of redemption, testimony, and teaching. Jesus was ministering in the power of the Holy Spirit Himself, fulfilling the divine plan of God to provide a sacrifice for our sins (John 3:16; 1 John 4:10). The Pharisees were attributing this to demonic activity."^ ^
Can believers commit it?
"No. A believer cannot commit the unforgivable sin. How can someone who has been born again (John 3:7), made a new creation (2 Cor. 5:17), and received eternal life (John 10:27-28) actually commit the unforgivable sin? He cannot. Jesus Himself said that we have eternal life, not conditional life: "My sheep hear My voice, and I know them, and they follow Me; and I give eternal life to them, and they shall never perish; and no one shall snatch them out of My hand." Besides, it says in 2 Cor. 5:17 that the Christian is a new creation in Christ. We are different, no longer slaves to the old nature (Rom. 6:14). We are regenerated by the Holy Spirit."^ ^