Well you have to look at the context of that. Jesus had cast some demons out of some people.
The Pharisees, the people who gave Jesus the most contention in the Bible and the people He was talking to in this incident, were a sect of legalistic, self-righteous people who believed in salvation through works, which is a false doctrine.
The Pharisees were especially concerned with Sabbath-keeping and attached so many man-made rules to it that it became a burden instead of the blessing God intended it to be, a day of rest after 6 days of hard-work in the field to rest and focus on God.
Jesus often confronted the Pharisees concerning their hypocrisy, because although they appeared moral on the outside, on the inside they were as wicked as anyone else. They would flaunt their prayers, giving, and fasting before men in order to be seen.
Another thing they would do is described in Mark 7:9-13 (KJV)
"9 And he said unto them, Full well ye reject the commandment of God, that ye may keep your own tradition.10 For Moses said, Honour thy father and thy mother; and, Whoso curseth father or mother, let him die the death: 11 But ye say, If a man shall say to his father or mother, It is Corban, that is to say, a gift, by whatsoever thou mightest be profited by me; he shall be free.12 And ye suffer him no more to do ought for his father or his mother;
13 Making the word of God of none effect through your tradition, which ye have delivered: and many such like things do ye. "
They would disobey the 5th commandment, to Honor their father and mother, by taking whatever financial support they would have otherwise have given their parents and made it a "gift to God" and then according to their traditions they wouldn't be allowed to help their parents anymore.
The Pharisees looked good on the outside, but were wretched on the inside.
Now, the Pharisees were expecting a Messiah who would come in, kill all the Romans and other gentiles, tell the Israelites how righteous they were, and set up an earthly kingdom where Israel would be the world power.(I'm not knocking Israel or anyone here, don't misunderstand me).
So when Jesus Christ, the Messiah, came, they rejected Him since He wasn't the conquering warrior they were expecting. Although they knew that Jesus was from God(see John 3 with the conversation between Nicodemus, a Pharisee, and Jesus), they decided to try and discredit Him, and were exceedingly angry when He told them about their hypocrisy.
Despite all the miracles and everything, they kept hardening their hearts and were so intent on discrediting Jesus that even though they knew that the miracles were from God, they attributed it to the evil one instead.
That hardness of heart was probably what Jesus was referring to. The Bible says that the Holy Spirit won't always strive with man(Genesis 6:3).
Hebrews 3:7-8 says "7 Wherefore (as the Holy Ghost saith, To day if ye will hear his voice,8 Harden not your hearts, as in the provocation, in the day of temptation in the wilderness:"
Acts 7:51 says "Ye stiffnecked and uncircumcised in heart and ears, ye do always resist the Holy Ghost: as your fathers did, so do ye."
Acts 7 deals with the trail of Stephen before the Sanhedrin, which would include Pharisees and other people, who, as verse 51 says, would always resist God's conviction upon them, thinking that they could make their own way into heaven.
The Bible says that it is possible to grieve and to even quench the Holy Spirit(Ephesians 4:30, 1 Thessalonians 5:19). The Holy Spirit does work patiently with people, but there does come a point where He will no longer strive with them. When does that time come? It's different, but the best bet is to not push your luck regarding God's patience.
In Romans 1 it talks about how God turning people over to a reprobate mind after resisting Him for too long.
The very fact that you're worried about this shows that you haven't committed the unpardonable sin of hardening your heart to the point of no return. If you did, you wouldn't be asking this.
