When our hearts are completely hardened against God.
Pharoah of Egypt is a good example.
Satan has also done this of course.
if you are worried that you may have committed the sin and can't be forgiven, don't be. If you are worrying about it, then you haven't committed it. If you are worried about it, then that is a sign that you have not committed it. If you had, you wouldn't be concerned.
Repentance is the sign that the Spirit is still with you.
"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."John 10:27,28
it is of course a final rejection of the Holy Spirit.
Mat 12:31 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.
It also when someone claims to be God when they are not.
Mat 26:64 Jesus saith unto him, Thou hast said: nevertheless I say unto you, Hereafter shall ye see the Son of man sitting on the right hand of power, and coming in the clouds of heaven.
Mat 26:65 Then the high priest rent his clothes, saying, He hath spoken blasphemy; what further need have we of witnesses? behold, now ye have heard his blasphemy.
Of course in this context, Christ is God.
John 10:33 The Jews answered him, saying, For a good work we stone thee not; but for blasphemy; and because that thou, being a man, makest thyself God.
Rev 13:6 And he opened his mouth in blasphemy against God, to blaspheme his name, and his tabernacle, and them that dwell in heaven.
The papacy claims to be God
Let me first give you how I understand the role of the Godhead in the salvation of humanity. Base on the Bible alone...
First, The Father, which is the provider.He is the King in the Old Testament, but what happened? The people of God rejected their king, the God of heaven and earth. Saul was anointed by Samuel to be king over Israel.(read 1Samuel)
Second, Jesus was born in Bethlehem. He is the King of the Jews, but of course the Lord of Lords and the King of Kings as well.(in the book of the Gospel)
Third, in the book of John 14, 16 when Jesus told his disciples that he will send the the Comforter which is the Holy Spirit.
Take note of this: these three are ONE. God the Father is appealing in the time of the Old Testament, Jesus repeated the call in the New Testament times. Now, the Holy Spirit as part of the Godhead is now appealing to us for the third time. He will teach us, remind the teaching that Christ taught when he was on earth and he is appealing too. He will convict is of our sins, to help us ready in the judgment time and lead is into all truth!
What is blasphemy? taking the place of God. Claiming to be God!
What is the role of the Spirit? Comforter. He convicts us of sin, and point us to the Truth!
So, now what is blaspheming the Holy Spirit? Taking the position of the Holy Spirit!
Remember the 3rd commandment... thou shalt not use the name of the Lord thy God in vain.....if you claim to be Christ followers but your doing the opposite? that's blasphemy!
To understand this verse you have to understand exactly what cause Jesus to say this.
John 16:13-15 (KJV) Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come. 14 He shall glorify me: for he shall receive of mine, and shall shew it unto you. 15 All things that the Father hath are mine: therefore said I, that he shall take of mine, and shall shew it unto you.