What is it to blaspheme against the Holy Spirit?
It can’t be to misuse his name, for whilst the misuse of God’s name is a sin, it is not unforgivable.
Looking at the situation Jesus was in, it appears more likely to be rejecting the work of the Spirit. For Jesus used the power of the Spirit in his ministry. Yet, the teachers of the law attributed the Spirit to be the work of Satan, and so rejected the work of the Spirit.
Jesus points out the absurdity of their argument. After all, how could Satan succeed if he fought against himself, especially on such a public level. Instead, the accusations were merely a wilful persistent rejection of the work of the Spirit. Today this culminates in the rejection of the Gospel, which the Spirit worked to establish in the word of God, which tells of all that Jesus Christ achieved for sinners.
On the flip side, Jesus remarks that everything else is forgivable. Such is the love and grace of God that any who repent of their sin and trust in Jesus Christ for forgiveness will be forgiven.
Much like the Israelites as they encountered their enemies in Canaan, we forget to consult God in our daily lives. Whilst sometimes we remember God when we’re unsure of what to do and we’re seeking guidance, we so easily forget God when we’re sure of what we’re doing, even though the matter is as important, if not more so.
This became apprent to the Israelites who were tricked into making an oath with a neighbour which God had forbidden them to do. Yet, even though this warns us of the folly of neglecting to consult God, God can use our mistakes for our benefit, and ultimately his benefit.
So we are not to despair if we make mistakes, but learn from them and consult God on all matters of significance, whether we think we know what we’re doing or not.