Hebrews 10:26-27

(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 is not referring to sin in general here but is specifically talking about apostasy. Apostasy means the renunciation of a religious or political belief. Once we learn of and accept the truth that is Jesus Christ, if we then turn away from it, we sin against the Holy Spirit, which Jesus said is the only unpardonable sin.

However having said that, if one turns away from the knowledge of Christ, only to return to the faith later in life, Christ will forgive him and he will be accpeted back into the loving arms of GOD.