Unlike the Bible, which is inerrant (no mistakes), apostles can make mistakes. Peter had to be corrected by Paul (Gal 2:11). At one stage Paul and Barnabas parted company over a disagreement (Acts 15:36-41). If the words of an apostle were the very words of God, these two incidents would not have been possible. The fact that they happened warns us that apostles are not perfect; they can (and do) make mistakes. Only the Bible is inerrant, thus only the Bible can be our ultimate authority.