“If the sacrifice is finished, isn’t Christ’s priesthood finished?”
No, why would it be? This is just a conclusion drawn from fallacious thinking. Let me lay out your argument and try to highlight the error:
A) A priesthood must make offerings in order to be valid.
B) Christ has His own priesthood.
C) If Christ’s priesthood is valid, they must make offerings.
D) The offering of Christ’s priesthood must be Christ’s sacrifice.
E) Christ’s sacrifice must never be finished, or His priesthood would become invalid.
First, you assume A without establishing it, but let’s assume that is a correct premise, just for expediency. The main error is in premise D, since you assume that the offering is Christ’s sacrifice, without actually establishing it. Without establishing that, your conclusion E is unsupported.
What is Christ’s priesthood, then?
And remember before you answer: Romans doesn’t just say he has a priesthood...he has a priesthood *of the order of Melchizedek*.