I have heard that some people claim that the Bible is the only source for Christian doctrines. How can this be so?
The Roman Catholic Church includes thirty-nine Old Testament and twenty-seven New Testament books of the Bible, the New Testament apocrypha books, and papal decrees when they determine doctrine. Therefore, it is true that they consider other sources than just the Bible when determining doctrine. But conservative or evangelical Protestant Christians look only to the Bible to determine doctrine, and to no other book. The following passages make it clear that Christians are not to look beyond the Bible.
But even if we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed! As we have said before, so I say again now, if any man is preaching to you a gospel contrary to what you received, he is to be accursed! (NASB) Gal. 1:8-9
I testify to everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues which are written in this book; and if anyone takes away from the words of the book of this prophecy, God will take away his part from the tree of life and from the holy city, which are written in this book. (NASB) Rev. 22:18-19
The sixty-six books of the Protestant Bible contain the words of God. All doctrine should be tested against the statements of the Bible. Any doctrine that does not agree with the Bible should be discarded.
You might be interested in reading the following, “Why do we accept the Bible?,” “What are the additional books of the Bible in the Catholic religion?” and “Should the Deuterocanon be included in the Holy Bible?” Sometimes understanding the Bible is difficult. That is why God encourages us to study the Bible. One cannot completely understand every aspect of the Bible without a life time of study (2 Tim. 2;15). We are commanded to study hard in order to correctly understand the Bible. Have fun studying!
Reference Links:Peace With God
Why do we accept the Bible?
What are the additional books of the Bible in the Catholic religion?
Should the Deuterocanon be included in the Holy Bible?
The Da Vinci Code