[Campion]

How much severer punishment do you think he will deserve who has trampled under foot the Son of God, and has regarded as unclean the blood of the covenant by which he was sanctified, and has insulted the Spirit of grace?

Clearly talking about a Christian who was justified and has fallen from grace. That means it's possible; Scripture doesn't waste time on impossibilities.

[Resettozero]

Scripture doesn't waste time on impossibilities.

Aramaic Bible in Plain English:
Luke 18:27
But Yeshua said, “Those things which are impossible with men can happen with God.”

[ealgeone]

Ephesians 1:13-14 tells us the believer is sealed by the Holy Spirit. The Word never tells us we are unsealed.