There is a lot of debate about this verse and others like it, but this is the way I understand it.

First, let’s get a little more context. This particular book of the bible is actually a letter written to a Jewish community who has come to believe Jesus was the coming king written about in their Bible for thousands of years. This is wonderful news for them, but it also puts a target on their back from other Jews (who happen to be in political power) who disagree. They now have to make a choice, abandon this idea for the sake of safety, or hold true to what they believe is true which has had beautiful results in their lives, despite the risk of mistreatment and death. The writer is appealing to their Hebrew bible (what the Christians call the old testament) to show that walking away is not what they really want.

Here are the verses again with what they are surrounded by:

By his death, Jesus opened a new and life-giving way through the curtain into the Most Holy Place. And since we have a great High Priest who rules over God’s house, let us go right into the presence of God with sincere hearts fully trusting him. For our guilty consciences have been sprinkled with Christ’s blood to make us clean, and our bodies have been washed with pure water.

Let us hold tightly without wavering to the hope we affirm, for God can be trusted to keep his promise. Let us think of ways to motivate one another to acts of love and good works. And let us not neglect our meeting together, as some people do, but encourage one another, especially now that the day of his return is drawing near.

Dear friends, if we deliberately continue sinning after we have received knowledge of the truth, there is no longer any sacrifice that will cover these sins. There is only the terrible expectation of God’s judgment and the raging fire that will consume his enemies. For anyone who refused to obey the law of Moses was put to death without mercy on the testimony of two or three witnesses. Just think how much worse the punishment will be for those who have trampled on the Son of God, and have treated the blood of the covenant, which made us holy, as if it were common and unholy, and have insulted and disdained the Holy Spirit who brings God’s mercy to us. For we know the one who said, “I will take revenge. I will pay them back.” He also said, “The Lord will judge his own people.” It is a terrible thing to fall into the hands of the living God.

Think back on those early days when you first learned about Christ [the messiah, coming king]. Remember how you remained faithful even though it meant terrible suffering. Sometimes you were exposed to public ridicule and were beaten, and sometimes you helped others who were suffering the same things. You suffered along with those who were thrown into jail, and when all you owned was taken from you, you accepted it with joy. You knew there were better things waiting for you that will last forever. So do not throw away this confident trust in the Lord. Remember the great reward it brings you! Patient endurance is what you need now, so that you will continue to do God’s will. Then you will receive all that he has promised. Hebrews 10:20‭-‬36

Hopefully this helps make this make more sense. This isn’t a blanket statement for all people of all time, it is about people considering abandoning their King for their own sake.

Also a little more about “the day of judgement” refered to here. This capital D “Day” is something referred to many times in the Hebrew bible. This is my understanding of the gist of it:

God doesn’t like the way us humans have decided to run the world. We kill and mistreat each other and the world around us. So he has decided to make everything right again and bring the world to peace. Great news if you are the one being mistreated, terrible news if you’re the one in power mistreating others. When the owner shows up, the managers are in trouble. The day of the Lord (the day of Yahweh)

The author of the letter is pleading with them to stay on the right side of history and not to align with the oppressors in power before God comes to make everything right.