"Let us hold unswervingly to the hope we profess, for he who promised is faithful. And let us consider how we may spur one another on toward love and good deeds. Let us not give up meeting together, as some are in the habit of doing, but let us encourage one another - and all the more as you see the Day approaching." NIV translation
Because Jesus has cleansed us, we have the hope of eternal life with Him. We can be confident of this because of the fact that He cannot lie. We are to hold on to His work that was performed on our behalf and not try to add the works of man. We are to share this hope with others bringing them to salvation and teaching them to be disciples of Him. Paul also reminds us that we are a part of the body of believers and so we need to be connected to the rest of the "church" especially as we get closer to the day of His return. There are many leaders who use this verse to call people to all kinds of "church functions" and make them feel guilty for not attending but that is not what Paul is saying. He is not saying that we need to join a "church" or denomination but instead we are to just get together regularly with fellow believers so that we can support and build each other up in the faith. The early church did this every day! Do you have daily fellowship with other believers?
"If we deliberately keep on sinning after we have received the knowledge of truth, no sacrifice for sins is left, but only a fearful expectation of judgment and of raging fire that will consume the enemies of God." NIV translation
Now, Paul gets to the heart of his message to the Jewish believers. He describes deliberate (willful) sin and that has been misunderstood and used to beat down Christians for way too long. We remember that Paul was speaking to Jewish Christians that were being told that they had to submit to the Law of Moses in addition to accepting the sacrifice of Jesus. The "willful sin" that he was speaking of is the act of allowing others to convince us to fall back under the law. While this may sound like the "religious" thing to do, by doing so we would be saying that Jesus' sacrifice was not sufficient. If that was true, then there would be no hope but only the dread of the coming judgment of God. That judgment is reserved for the "enemies of God" so, who are they? They are those that reject the plan of salvation through Jesus Christ and trust in their own works. That, my friends, is why so many Christians walk around like slaves instead of the royalty that we are.