There are three ways I’ve heard this can be answered. First, the Blaspheming of the Spirit could be something only those witnessing Jesus’ earthly ministry are capable of committing. The incarnation is the unique presentation of God through Jesus, and, as such, is unrepeatable; further, the Holy Spirit led Jesus, worked in Jesus, and testified to men through and in Jesus’ earthly healing ministry. Those witnessing Jesus’ life during this time were in an incredible position of benefit because they could see the presentation of the Father in the Son and also see the work of the Spirit with Jesus while the Spirit Himself was trying to persuade those witnessing these mighty works. We are not in the same position as those who originally saw Jesus’ works while He was incarnated; they had immediate access with their very eyes, but we have mediate access through the testimony of the apostles.
Second, the Blaspheming of the Spirit could be disbelieving in Jesus, period, no matter when you live. There is a principle in theology that is based on Scripture: God is known only through God. Hence, only the Father is known through the Spirit, or only the Father is known through the Son in the Spirit. The point is that Scripture is united in the affirmation that it takes God to know God. Since the principle ministry of the Spirit is to testify to the divinity, message, and work of Jesus (the Son), to blaspheme the Spirit is to deny the only access one has to being restored to God. There is no other access to salvation but by one Man and in one Spirit. To blaspheme means to disrespect or to profane, that is, to discredit what is sacred of its sanctity; but it is the Spirit’s holiness/sanctity that makes Him capable of uniting humanity to God because only God is holy. Therefore, to blaspheme the Spirit is, to one’s own mind, to deny the Spirit the very dignity (being holy) that is central to the Spirit’s work of uniting someone to God. To blaspheme the Spirit is to rob Him of the very quality that could unite us to God: because God can only be known through God, that is, the Holy One can only be known through the Holy Spirit.
Lastly, the original blaspheming of the Spirit was the accusation that what the Spirit was doing in Jesus in terms of the miracles was actually being done by demons. Since the Spirit can do miracles today, if we attribute those miracles to demons, then we are in danger of blaspheming the Holy Spirit. This is perhaps the most frightening of the three because Scripture teaches that demons can do miracles and because the recent charismatic movement’s focus on miracles, which has been criticized by many non-charismatic Christians: John MacAuthur comes to mind immediately. This option is also one that many more could commit even if supposedly saved.
I have not quite landed on an option yet, but I have been giving serious thought about the third because of the threat it could pose my soul.