What is blasphemy of the Holy Ghost? [Edit to add Scripture] [b]Matthew 12:22-32 NASB[/b] 22 Then a demon-possessed man who was blind and mute was brought to [a]Jesus, and He healed him, so that the mute man spoke and saw. 23 All the crowds were amazed, and were saying, “This man cannot be the Son of David, can he?” 24 But when the Pharisees heard this, they said, “This man casts out demons only by [b]Beelzebul the ruler of the demons.” 25 And knowing their thoughts Jesus said to them, “[c]Any kingdom divided against itself is laid waste; and [d]any city or house divided against itself will not stand. 26 If Satan casts out Satan, he [e]is divided against himself; how then will his kingdom stand? 27 If I by [f]Beelzebul cast out demons, by whom do your sons cast them out? For this reason they will be your judges. 28 But if I cast out demons by the Spirit of God, then the kingdom of God has come upon you. 29 Or how can anyone enter the strong man’s house and carry off his property, unless he first binds the strong man? And then he will plunder his house. 30 He who is not with Me is against Me; and he who does not gather with Me scatters. [b] 31 “Therefore I say to you, any sin and blasphemy shall be forgiven people, but blasphemy against the Spirit shall not be forgiven. 32 Whoever [g]speaks a word against the Son of Man, it shall be forgiven him; but whoever [h]speaks against the Holy Spirit, it shall not be forgiven him, either in this age or in the age to come.[/b]

Personally, I would say the renouncing (or denouncing, depending on the circumstance) of Jesus Christ as G-d, and of G-d as ruler of the universe, and Creater of all things.

http://carm.org/what-blasphemy-holy-spirit-can-christian-commit-it [quote]Jesus mentions a sin that is unforgivable in Matt. 12:31-32 and calls it blasphemy of the Holy Spirit. But what exactly is this unforgivable sin? For that, we need to look at the context. Matt. 12:22-32 says, "Then there was brought to Him a demon-possessed man who was blind and dumb, and He healed him, so that the dumb man spoke and saw. 23And all the multitudes were amazed, and began to say, "This man cannot be the Son of David, can he?" 24But when the Pharisees heard it, they said, "This man casts out demons only by Beelzebub the ruler of the demons." 25And knowing their thoughts He said to them, "Any kingdom divided against itself is laid waste; and any city or house divided against itself shall not stand. 26"And if Satan casts out Satan, he is divided against himself; how then shall his kingdom stand? 27"And if I by Beelzebub cast out demons, by whom do your sons cast them out? Consequently they shall be your judges. 28"But if I cast out demons by the Spirit of God, then the kingdom of God has come upon you. 29"Or how can anyone enter the strong man's house and carry off his property, unless he first binds the strong man? And then he will plunder his house. 30"He who is not with Me is against Me; and he who does not gather with Me scatters. 31"Therefore I say to you, any sin and blasphemy shall be forgiven men, but blasphemy against the Spirit shall not be forgiven. 32"And whoever shall speak a word against the Son of Man, it shall be forgiven him; but whoever shall speak against the Holy Spirit, it shall not be forgiven him, either in this age, or in the age to come," (All Scripture quotes are from the NASB). Let me review this section briefly. In verse 22, Jesus healed a blind and dumb man. The Pharisees accuse Jesus of casting out demons by the power of "Beelzebub, the ruler of the demons" (v. 24). Jesus responds by saying that a kingdom divided will fall (vv. 25-28) and how the devil must first be bound before you can plunder his house (v. 29). In verses 31-32, He states that blasphemy against the Spirit will not be forgiven in this age or the age to come. By simply looking at the context it becomes apparent that blasphemy of the Holy Spirit is saying that Jesus did His miracles by the power of the devil. This is unforgivable. [...] He did His miracles by the power of the Holy Spirit who had come upon Him at His baptism. The Pharisees - who knew that Jesus' miracles validated His words and ministry (see John 11:45-48) - were attempting to discredit Jesus' Messiahship by saying that His works were by the devil and not by the Holy Spirit. Therefore, when the Pharisees accused Jesus of casting out demons by the power of Satan, they were blaspheming the Holy Spirit by whom Jesus performed His miracles. This is unforgivable because it struck at the very heart of the redemptive work of God in Christ.[/quote] I encourage you to read the full article via the link provided to better understand this.