How do Protestants understand the "unforgivable" sin?

Scripture refers to an unpardonable sin in [Matthew 12:31–32](https://www.biblegateway.com/quicksearch/?quicksearch=Matthew+12:31-32&qs_version=ESV) , [Mark 3:29–30](https://www.biblegateway.com/passage/?search=Mark+3:29-30&version=ESV) , and [Luke 12:10](https://www.biblegateway.com/passage/?search=Luke+12:10&version=ESV) , as well as [Hebrews 6:4–6](https://www.biblegateway.com/passage/?search=Hebrews+6:4-6&version=ESV) . There's also "sin that leads to death" ([1 John 5:16–17](https://www.biblegateway.com/passage/?search=1+john+5:16-17&version=ESV)) . Given the [Protestant understanding of sin](https://christianity.stackexchange.com/q/41820/21576) , **what is an overview of the doctrines they develop from these passages?** To Protestants, what is the unforgivable sin? Related, but not tradition-specific: https://christianity.stackexchange.com/q/3334/21576