Albert Barnes on Hebrews 10:9

Then said he. In another part of the passage quoted. When he had said that no offering which man could make would avail, then he said that he would come himself.

He taketh away the first. The word "first" here refers to sacrifices and offerings, he takes them away; that is, he shows that they are of no value in removing sin. He states their inefficacy, and declares his purpose to abolish them.

That he may establish the second. To wit, the doing of the will of God. The two stand in contrast with each other; and he shows the inefficacy of the former, in order that the necessity for his coming to do the will of God may be fully seen. If they had been efficacious, there would have been no need of his coming to make an atonement.