But this man - The Lord Jesus. The word “man” is not in the original here. The Greek is literally “but this;” to wit, this priest. The apostle does not state here whether he was a man, or a being of a higher order. He merely mentions him as a priest in contradistinction from the Jewish priests.
After he had offered one sacrifice for sins - By dying on the cross. This he did but once; this could not be repeated; and need not be repeated, for it was sufficient for the sins of the world.
For ever sat down - That is, he sat down then to return no more for the purpose of offering sacrifice for sin. He will no more submit himself to scenes of suffering and death to expiate human guilt.