Again you are reading this out of context. The context is: "Think how much more terrible the punishment will be for those who have trampled on the Son of God and have treated the blood of the covenant as if it were common and unholy. Such people have insulted and enraged the Holy Spirit who brings God's mercy to his people" (v.29). They were never born-again to begin with just as you are not born-again since you rely on self-strength to be saved and to keep yourself allegedly saved. This is why you boast you never sin which is a lie, for you have several false teachings in your profile.

You're reading legalistically. The reason you can't understand this is because "we impart this in words not taught by human wisdom but taught by the Spirit, interpreting spiritual truths to those who possess the Spirit" (1 Cor. 2.13). You don't have the Holy Spirit.

Picture a circle and its center. In the center are the seemingly least sinful people and outer edges are the most evil people like the Antichrist. Once a person is born-again they are taken off that circle altogether and placed in a new circle with Jesus at the center. This circle is not as big a the other circle, but it still has every every kind of Christian, some sinning more than others and some more selfish than others. The circle is smaller because they are circumscribed by not doing some things those in the larger circle would because of their new life. The people in the smaller circle all have eternal life which can never be lost, but only the inner circle within the smaller circle will receive the reward of returning with Christ during the millennium. They all could have been in the inner circle, but sadly many do not. So they go to another place which is another circle of "outer darkness" outside the light of rewards of reigning with Christ during the millennium.

To show this inner circle more clearly, we read,

"Then I saw thrones, and seated on them were those to whom judgment was committed. Also I saw the souls of those who had been beheaded for their testimony to Jesus and for the word of God, and who had not worshiped the beast or its image and had not received its mark on their foreheads or their hands. They came to life, and reigned with Christ a thousand years" (Rev. 20.4).

Do all Christians fit this description? Of course not. Notice the 3 groups in view here.