Answer:
There are several places that talk about this, such as this verse:
Revelation 21:8 But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death.
However, this is in reference to those who do not repent and seek God’s forgiveness. David was actually a murderer (he conspired to have Bathsheba’s husband murdered) and so was the Apostle Paul (killing Christians including being part of Stephens stoning.).
The only unforgivable sin is blasphemy of the Holy Spirit, and this is a willful and continual rejection of all of God’s truths even when you know better. Jesus warned the Pharisees of this because even though they could see Jesus was fulfilling scripture and doing His Fathers work they attributed it to the devil.
Good question,
Pastor Brad