Don't Christians sin after baptism?

As long as we dwell in this flesh, we have a lot of opportunities to commit sins. Like it or not, there are a host of needs to be satisfied: hunger, thirst, rest, sex, curiosity, money, honour, power etc. As long as we satisfy these needs in an appropriate manner, not too little, not too much, though most of the time, it is safer to opt for the deficient side. Let's face it. This is reality. Christians are not immune to temptations and sins after baptism. They need to work really hard not to commit sins. Therefore, it is rather disturbing to read the following verse from the Hebrews.
For if we sin deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins,
but a fearful prospect of judgment, and a fury of fire which will consume the adversaries (Hebrews 10:26-27).
In trying to demonstrate the superiority of Jesus' sacrifice over all the daily, yearly offerings of sacrifice prescribed by the Torah, the author of Hebrews argued that Jesus did this once and for all (Hebrews 7:27). It was not necessary for Jesus to offer himself repeatedly. Therefore, his sacrifice is superior.
From what the author wrote above (10:26), it seems that Jesus' sacrifice does not cover the sins committed after baptism, after receiving the knowledge of the truth. In other words, Jesus' sacrifice does not cover all. It is incomplete and contradicts his previous claim (7:27). It also contradicts what Christians believe and practise. Protestants pray to God to obtain forgiveness of their sins. Catholics go to confessions, to receive the Sacrament of Reconciliation to have their sins forgiven. So, how should we understand Hebrews 10:26? Or did the author make a mistake?
Obviously, the author wanted to warn against the abuse of freedom granted in the New Covenant. With greater freedom comes greater responsibility. We partakers of the New Covenant have to keep ourselves free from the slavery of sins. We have the Holy Spirit to help us transcend our lowly humanity. There remains tensions between the call to sanctification and our fleshly desires. St. Paul expresses it in the two laws.
I do not do the good I want, but the evil I do not want is what I do (Romans 7:19).
This tension remains as long as we live. The author of Hebrews resorts to the fearful prospect of judgment to warn against the abuse of God's grace after baptism.
Vengeance is mine, I will repay (Hebrews 10:30, Deuteronomy 32:35a).
The author of Hebrews also resorts to what his readers have done well.
But recall the former days when, after you were enlightened, you endured a hard struggle with sufferings, sometimes being publicly exposed to abuse and affliction, and sometimes being partners with those so treated.
For you had compassion on the prisoners, and you joyfully accepted the plundering of your property, since you knew that you yourselves had a better possession and an abiding one (Hebrews 10:32-34).
Persecutions are good in purifying our evil tendencies. Joyfully accept them to be purified.

Dear Lord, You chasten those You love. May our hope in heavenly inheritance sustains us along our way home. Amen.

Appendix:
dies without mercy at the testimony of two or three witnesses.
ἐπὶ δυσὶν ἢ τρισὶν μάρτυσιν ἀποθνῄσκει· (Hebrews 10:28b)
ἐπὶ δυσὶν μάρτυσιν ἢ ἐπὶ τρισὶν μάρτυσιν ἀποθανεῖται ὁ ἀποθνῄσκων (Deuteronomy 17:6a).

Vengeance is mine, I will repay. ἐμοὶ ἐκδίκησις, ἐγὼ ἀνταποδώσω. (Hebrews 10:30a)
ἐν ἡμέρᾳ ἐκδικήσεως ἀνταποδώσω (Deuteronomy 32:35a)
The Lord will judge his people. κρινεῖ κύριος τὸν λαὸν αὐτοῦ. (Hebrews 10:30b)
ὅτι κρινεῖ κύριος τὸν λαὸν αὐτοῦ (Deuteronomy 32:36a)

For yet a little while,
ἔτι γὰρ μικρὸν ὅσον ὅσον, (Hebrews 10:37a)
μικρὸν ὅσον ὅσον, (Isaiah 26:20)
and the coming one shall come and shall not tarry;
but my righteous one shall live by faith, and if he shrinks back, my soul has no pleasure in him. ὁ ἐρχόμενος ἥξει καὶ οὐ χρονίσει
ὁ δὲ δίκαιός μου ἐκ πίστεως ζήσεται, καὶ ἐὰν ὑποστείληται, οὐκ εὐδοκεῖ ἡ ψυχή μου ἐν αὐτῷ. (Hebrews 10:37b-38)
ὅτι ἐρχόμενος ἥξει καὶ οὐ μὴ χρονίσῃ.
ἐὰν ὑποστείληται, οὐκ εὐδοκεῖ ἡ ψυχή μου ἐν αὐτῷ, ὁ δὲ δίκαιος ἐκ πίστεώς μου ζήσεται. (Habakkuk 2:3b-4)