The author of Hebrews places emphasis on the finished work of Christ and refers to it as once and for all.

Contextually, the putting away of sins and the once for all reference(s) is in regards to superiority of the New Covenant in contrast to the Old, and the permanency of the finished work of Christ in contrast to the imperfect atonement under the Law which was merely as shadow of things to come .

Notice the following from Hebrews 7:

Wherefore he is able also to save them to the uttermost that come unto God by him, seeing he ever liveth to make intercession for them. For such a high priest became us, who is holy, harmless, undefiled, separate from sinners and made higher than the heavens; Who needth not daily, as those high priests, to offer up sacrifice, first for his own sins, and then for the people’s: for this he did once, when he offered up himself. For the law maketh men high priests which have infirmity; but the word of the oath, which was since the law, maketh the Son, who is consecrated for evermore. ~ Hebrews 7:25-28 

Notice also the following from Hebrews 10:

By the which will we are sanctified through the offering of the body of Jesus Christ once for all. And every priest standeth daily ministering and offering oftentimes the same sacrifices, which can never take away sins: But this man, after he had offered one sacrifice for sins forever, sat down on the right hand of God. ~ Hebrews 10:10-12

The blood of animals could not purify man’s conscience in the sight of God and those offerings were not sufficient to provide lasting atonement for all sin, therefore atonement had to be made annually for the nation and daily for individuals. Furthermore, those sacrifices could not take away sin and cleanse the conscience from guilt. Jesus’ sacrifice, however, provided atonement once and for all. His sacrifice is perfect and avails forever.

Christ’s sacrifice being “once for all” does not mean that God can’t see our sins. It does not mean that we do not need to repent when we know we have sinned. It simply means that there is no longer any need for offerings for sins. Jesus’ sacrifice will cleanse us and keep on cleaning us when we sin if we continue in faith towards Christ.

Jesus is a priest forever after the order of Melchisedek (He is both priest and King seated at the right hand of the Majesty on High). There will never, ever, be a need for another sacrifice or another priest. In fact, the Book of Hebrews tells us that if we go on sinning (living in sin and rejecting Jesus) after we have received the knowledge of the truth there is no more offering for sin.

Jesus’ ministry as our priest is unlike those who were ordained under the Law:

He does not need to offer sacrifices every day. They did this first for their own sins and then for the sins of the people. But Jesus did this once for all when he offered himself as the sacrifice for our sins. ~ Hebrews 7:27