The unforgivable sin: what is it, and have I committed it? These two questions pop up a lot in the Christian faith, especially among "baby Christians". There's a lot of debate about whether or not it can even be committed in the modern day.
So what is the unforgivable sin? The concept of an unforgivable sin comes from two Bible verses: Matthew 12:32 and Mark 3:29. In both cases, Jesus states outright that those who blaspheme the Holy Spirit will never be forgiven. The unforgivable sin, therefore, is blasphemy against the Holy Spirit.
But what does this mean? In context, the Jews, seeing Jesus' miracles, came up with an alternative explanation for how He did them: Jesus was in communion with demons. But the unforgivable sin isn't so simple. It's actually twofold. The first half of it can still be committed today. Put simply, it is building up such a high standard of evidence that no amount of evidence could convince you that Jesus is Lord. Many unbelievers still do this today. They can be forgiven, but they never will be because they don't allow themselves to be. If belief is required for salvation, a refusal to allow yourself to be convinced prevents you from receiving that salvation.
But the second layer of the unforgivable sin is not available to us today. It requires the physical presence of Jesus performing His miracles. In the Old Testament, the foundation for belief in the Messiah was built up pretty strong. Specifically, Isaiah prophesied the exact type of miracles Jesus would perform: Healing blindness, deafness, and lameness (Isaiah 35:5-6). If you're expecting the Messiah to come and heal the blind, deaf and lame, and a guy comes along healing the blind, deaf and lame, the correct response is to worship Him, not to reject your own scriptures and declare that the only reason He looks like the Messiah is through the power of the anti-Christ!
If you're a Christian, and you're worried you've committed the unforgivable sin, don't worry, you haven't. It's actually impossible to commit it now Jesus isn't performing miracles, and the fact that you are even concerned about having committed it proves you can't have. If you're not a Christian, I'd like to ask a question: Why? It's one thing to ask for evidence. Indeed, the Bible commands that you do so (1 Thessalonians 5:21). But beware, because while the unforgivable sin isn't possible in its entirety, it's still possible to set your standard of evidence so high that you can never accept the forgiveness available to you.