The ironic thing about this passage is that the greater message here is about the FORGIVENESS of God. What we see in Christ's discourse on the "eternal sin" is a severe warning (gracious in nature) about the consequences of "un-forgiveness". The scary part of this is that Jesus (the evangelist) was preaching this message to those who would confidently assert their right-standing with God.

Additional Scripture References:

Matthew 12:27-30, 2 Peter 3:9, 1 Timothy 2:4, Ezekiel 18:23, Psalm 103:12, Mark 1:4, Mark 1:15, John 16:8, Hosea 4:17, Psalm 81:12, 2 Peter 1:8-10, Romans 8:13, Col. 3:5, Matt. 12:33-37