Here our Lord plainly teaches that every sin and blasphemy can be forgiven except the blasphemy against the Holy Spirit (Mark 3:29). But, what exactly is this sin? To answer that difficult question, we will first consider two distinct possibilities: (1) either the Pharisees and scribes did blaspheme the Holy Spirit in this context or (2) they did not blaspheme the Holy Spirit in this context. Let us carefully analyze both positions--one today and another tomorrow--and determine which one is more logical.
There are those who believe that the Pharisees and scribes did not blaspheme the Holy Spirit in this passage. To hold this view is essentially to have Jesus issuing a warning to them for the future. Or, in other words, Jesus basically told them: "Today, you have sinned by blaspheming the Son of Man (cf. Matt. 12:32). You have spoken out against Me, not from ignorance, but from hard-heartedness and open rejection of the truth. Religious leaders, you have witnessed a great miracle today, but you don't want to believe in Me and the power of God Almighty working through Me." If this is the correct interpretation, then the Pharisees could be forgiven since they had only blasphemed Jesus at this time and not the Holy Spirit.
But, this raises some questions: What is the difference between blaspheming Jesus and blaspheming the Holy Spirit? Why is forgiveness possible for the first type of blasphemy but not the second? Some have suggested the following answer. It is as if the Lord is saying: "You might reject Me today and blaspheme My present works, and yet you could still be forgiven because there is still more truth to be revealed through the Holy Spirit. But, if you blaspheme and reject the message of the Holy Spirit when He comes and speaks through My apostles and the inspired written word, then there can be no forgiveness--not in this world and not in the world to come! The Holy Spirit's message will be heaven's last offer of pardon to you. If you curse it and reject it, then you will never be forgiven."
Consider the following illustration that supports the idea that the Pharisees did not blaspheme the Holy Spirit in this context. Let's suppose that there is a man in a boat floating down a river. Down stream there is a deadly waterfall that he is unaware of. Some distance up the stream, there is a man who sees him headed for certain danger. He shouts a warning and offers assistance, but the warning is refused. The man drifts further down the river. Later, a second man also calls out and offers help. His warning is likewise rejected. The man continues down the stream, offended by those who have tried to help him. After all, he knows what he is doing (or so he thinks)! Just before reaching the brink of the falls, a third man sees him and shouts out to him to turn back before it is forever too late. But, this final warning is also refused, and the man plunges to his death. Three offers of assistance were made; acceptance of any of them would have delivered the man from destruction. The doomed man might have rejected the warnings of the first two men (and even spoken evil against them--blasphemy) and later have been saved by heeding the third warning. However, his refusal of the last offer sealed his fate.
In a similar manner, the Spirit's message, through the apostles, would be the final offer of pardon. One might have rejected John the baptizer's message and Jesus' message and still have been saved later by responding to the apostles' inspired words (given by the Holy Spirit). However, to reject that message would be to reject one's final warning (and essentially blaspheme the Spirit of God). Thus, if the Pharisees rejected the last chance of pardon that would soon be proclaimed, then their condemnation would be fatal--there would be no forgiveness for them if they later reject the fully revealed gospel!
There is definitely some evidence, though speculative, to support the view that the Pharisees did not blaspheme the Holy Spirit here. However, I believe the second interpretation fits the context better. We will consider this alternative interpretation tomorrow.