Hebrews 10:10 says that Jesus’ offering sanctified us once for all. Verse 14 says that those of us who have been sanctified by Jesus’ offering were also perfected forever.
The way to understand this wonderful truth is to recognize that when we are born again, we receive new spirits that are created in righteousness and true holiness (Ephesians 4:24). The new, born-again spirit is as holy and pure as Jesus is (1 Corinthians 6:17 and 1 John 4:17) because it is His Spirit (Galatians 4:6). The moment we believe, we are sealed (vacuum packed), and sin cannot penetrate that seal (Ephesians 1:13).
When Christians sin, their bodies and souls participate and are affected by that sin, but their spirits are sealed and retain their holiness. That’s the reason Hebrews 10:14 says they are perfected forever. That perfection is in their born-again spirits (Hebrews 12:23).