And let us consider one another in order to stir up love and good works, not forsaking the assembling of ourselves together, as is the manner of some, but exhorting one another, and so much the more as you see the Day approaching. For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries.
- Hebrews 10:24-27 (NKJV)
I believe Hebrews 10:26 is talking about the unforgivable sin.
If we sin willfully after we have received the knowledge of the truth, this is unforgivable.
But consider… How many individuals have been taught the truth? The truth is found in God's Word, every single verse of it, but not everyone has read it. For this reason, it is my opinion that only a Christian—one who has complete understanding of God's Word and who has the Holy Spirit abiding inside him that can commit the unpardonable sin. A Christian will not be pardoned for refusing to allow the Holy Spirit to use him (speak through him). We are in the latter days, and the Day is approaching. Let us not deny the Holy Spirit.
"You will be brought before governors and kings for My sake, as a testimony to them and to the Gentiles. "But when they deliver you up, do not worry about how or what you should speak. For it will be given to you in that hour what you should speak; "for it is not you who speak, but the Spirit of your Father who speaks in you.
- Matthew 10:18-20 (NKJV)