Let's back up a little further in Hebrews 10, and also look at the verses beyond what I quoted above. I'll insert my own comments and opinions in
red: (Feel free to point out my errors if you perceive any.)
.19 Therefore, brothers and sisters, since we have confidence to enter the Most Holy Place by the blood of Jesus, 20 by a new and living way opened for us through the curtain, that is, his body, (Paul is talking about Christians here, not Jews)
21 and since we (Christians) have a great priest over the house of God, (Jesus) 22 let us draw near to God with a sincere heart and with the full assurance that faith brings, having our hearts sprinkled to cleanse us from a guilty conscience and having our bodies washed with pure water. 23 Let us hold unswervingly to the hope we profess, for he who promised is faithful.
24 And let us consider how we may spur one another on toward love and good deeds, 25 not giving up meeting together, as some are in the habit of doing, but encouraging one another—and all the more as you see the Day approaching. (Paul is encouraging Christians to do God's will; avoid sin)
26 If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, 27 but only a fearful expectation of judgment and of raging fire that will consume the enemies of God. (Paul warns what will happen if you fail to do God's will--that is, deliberately keep on sinning)
28 Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses. 29 How much more severely do you think someone deserves to be punished who has trampled the Son of God underfoot, who has treated as an unholy thing the blood of the covenant that sanctified them, and who has insulted the Spirit of grace? (Paul compares judgement for Christians to judgement under the law, and states that Christians will be judged more severely for trampling the Son of God underfoot)