Hello Rebecca, I have responded to a similar question before in greater detail. If you are interested, I recommend reading it, as well: Q10. Can you lose your salvation or are you eternally secure?

Regarding your question, Hebrews 10:26-27 reads:

For if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a fearful expectation of judgment, and a fury of fire that will consume the adversaries.

Theologians consider this verse to be speaking of apostasy, the intentional falling away or deliberate rejection of Christ. The true nature of apostasy is complex, but often refers to those on the verge of saving faith, but nevertheless reject or deny its power. In fact, Jesus even refers to the condition of the apostate’s heart in the Gospels in the Parable of the Sower (Matthew 13:1-9, 18-23). An apostate who lives in sin, pleasurably and deliberately, is effectively “crucifying once again the Son of God to their own harm and holding him up to contempt” (Hebrews 6:6). By continually sinning and not repenting, we effectively say that Christ is powerless through our words and actions. Apostates, then, have “knowledge of the truth” but do not fully apply the truth in their lives. Their sin is willing, deliberate, intentional, habitual, and careless, even indifferent. For what does John say? “No one born of God makes a practice of sinning, for God’s seed abides in him; and he cannot keep on sinning, because he has been born of God.” (1 John 3:9)

Despite this, that does not mean apostates cannot repent at any time. Everyone is free to repent. And we should pray for the faithful return of anyone who has fallen away! And for apostates who return to the faith, Paul encourages the Church to forgive, comfort, and love them, so they will not be overwhelmed with excessive sorrow (2 Corinthians 2:6-8; James 5:15-16, 19-20; Jude 1:3, 20-23a).

Our assurance is in Christ. Holdfast to His testimony! God saves, not us. I hope that helps. God Bless, Rebecca!