The basis for the statement in the BoC that the Pope is the Antichrist is based upon the facts that when the Pope is speaking ex cathedra, with supposed infallibility, he is standing in the place of Christ, and because the Pope teaches against the scriptural doctrine of justification by grace alone, through faith alone, in Christ alone, he teaches false doctrine from the place of Christ. These are the scriptural descriptions of the Antichrist.
It is important to remember that Lutherans do not buy into the whole millennialist schtick regarding the Antichrist, as portrayed in Left Behind or otherwise.