The one and only unforgivable sin is blasphemy against the Holy Spirit (Matthew 12:22-37). The only account we have of this sin is that some Pharisees did it - they saw a miracle of God, but since Jesus did it, and they hated Jesus, they said that Jesus tapped into Satan, rather than God, to do it. Jesus explained why this accusation was both false and nonsensical, which highlights the sheer hatred and rejection of God that it takes to commit this sin. And of course, they knew better (John 3:1-2). They saw his miracles.
Satan likes to deceive Christians into thinking they did it. But since our hearts are not where the Pharisees' hearts were at, and God knows this, we don't do it. We have the Holy Spirit in us, convicting us of sin (John 16:8). If somebody did it, that person wouldn't care - at least until the Last Judgment. And remember, the Holy Spirit represents an eternal investment that God has made in us (2 Corinthians 1:21-22, 1 Peter 1:23). Why would God plant that imperishable seed into someone who he knew would just fall away? Remember, the seed is imperishable, not perishable. Those who appear to lose their salvation, according to John, never had it (1 John 2:19). Satan knows this. He knows he can't take God's children through force or guile (John 10:28-30). So the least he can do is damage control by making our lives miserable, by tempting us so that we will not effectively be used by God to put even more souls out of his reach, etc. This "have I committed the unforgivable sin" thing is just part of his bag of tricks. Don't be deceived by it.