On this very blog, we have a clear example of the Unforgivable Sin.  In fact, the clearest ever offered here.  Alex said this:

It needs to be mentioned that the definition of nature is what science can measure and the reason we call your god’s being and doings supernatural is that when we measure, there is no god there, only natural explanations such as physics and chemistry. (source)

The Unforgivable Sin, from Mark 3:22-30 or Matthew 12:22-32, is the denial of the Spirit moving in our world.  It is through the movement of the Spirit that we can see evidence of God acting in our world.

So, a little context.  I deny the categories of natural and supernatural.  Alex is saying that when we measure the doings of God, we find no God, just natural movements.

The first problem is that our instruments aren’t going to measure or detect God, who exists outside of the time and space we know how to measure.  Instead, what we’re going to see are the effects that God creates, which are accomplished by the Holy Spirit.  This is the evidence of God.

Denying that what we have seen is the movement of the Spirit is the Unforgivable Sin.

For example, Alex looks at biochemistry.  The amazing complexity and well-oiled interactions of the various systems of our bodies, the ability of our bodies to obtain the raw ingredients our cells need to produce energy in the foods we eat and the drinks we consume all bear evident marks of design.  The well-defined stages of growth humans go through, the inherent curiosity to learn and flourish, shared ability to define morality, to know what is is not what ought to be; these are the hallmarks of a being who can impart these things to us.

Alex, however, looks a this design and says, “Nah, random mutation acted on by natural selection — not a personal, intelligent force — created this.”

And that, my friends, is the Unforgivable Sin in a nutshell.  The Pharisees saw the work of the Spirit in Christ as he drove out demons and cured disease, and they attributed it to Satan.  Alex sees the work of God in chemistry and biology and attributes it to chance and natural laws.  Both deny the Spirit’s efficacy, and both have severe eternal consequences.