When that denomination led by the Bishop of Rome took away the blood of Christ from the laity, it demonstrated its ability to err. Here Luther argues that the only fully trustworthy and infallible man is Christ Jesus. To Him alone are we commanded to listen:

The fallibility of the church has been borne out by the fact that it accepted, and approved of, the pope’s proposal to restrict Holy Communion for the laity to one kind. The church did not resist this move or contend against it. But Christ nonetheless preserved the church by reason of her daily petition for “the forgiveness of sins” in the Christian Creed, as we still pray in the Lords Prayer: “And forgive us our trespasses as we forgive those who trespass against us.” Therefore regard the church as holy. Nevertheless, I do not trust myself to her; for the spectacle of the church in the papacy prevents me from doing so. If they teach Christ properly, I, too, shall believe and trust her; for in Christ I know no sin, no error, no lie. He alone is the Truth and the embodiment of the Father’s will. For of Him the Father says: “This is My beloved Son, with whom I am well pleased; listen to Him” (Matt. 17:5). If I were to give ear to anyone else, error would certainly come in.

LW 22, pp258f