This sin refers to "apostasy" meaning a deliberate rebellion against God. This references people who were Christians and decided they didn't need God and began living a sinful lifestyle.

In regards to sinning after we know it's wrong, our natural selves will always continue to sin, whether by words, actions or thoughts no matter how much we love God. The difference is whether we are trying our best to live a holy life or focused on our selfish desires.

Jesus will always forgive a believer who repents of their sin and honestly tries to live a holy life.