John Macquarrie writes concerning Matt. 16:18, etc.
" 'The powers of death shall not prevail against it.' But this is not to claim infallibility for the Church. It is indeed to assert a measure of authority for it and to declare its normal superiority over individual judgement. But since the Church is at any given time less than the kingdom, its authority is not absolute and, as has been shown already, must be counterbalanced by the authority of scripture and also that of reason. If the statement that General Councils 'may err and sometimes have erred' seems to be somewhat negative, it is not to be taken as implying in the slightest degree any disrespect for the Church, but is simply an acknowledgement that the Church, understood as process rather than fulfillment, and so less than Christ and less than the kingdom, does not have absolute authority 'even in the things pertaining to God.' " (Principles of Christian Theology, Page 390. 2nd Ed.)
And as one of my heroes from the 17th century said: "[it is] true and certain that the church can stray from the path and choose error instead of truth," thus the Bible's authority (AUCTORITAS) "is far superior to the authority of the church . . ." (Cyril Lucaris qt. in Jaroslav Pelikan's The Christian Tradition 2:284-285).