The Bible is enough. It is, but a two book approach has chipped away at biblical Christianity. What is the two book approach? Someone, who says he believes in it, writes:
Theology and . . . . are both sciences in their own right, stand legitimately on their own foundations, read carefully are the two books of God’s Revelation. . . . Wherever truth is disclosed it is always God’s truth. Whether it is found in General Revelation or Special Revelation, it is truth which has equal warrant with all other truth. Some truth may have greater weight than other truth in a specific situation, but there is no difference in its warrant as truth.
Generally, the language "two book approach" has been applied to so-called Christian psychology, but I see it as a general rejection of sola scriptura and the sufficiency of scripture. This has become the norm today.
I'm calling this a primer, because I want to lay out briefly the major areas we are witnessing this. Like covetousness is called idolatry in Colossians 3:5, the two book approach is faithlessness. It isn't sola scriptura when you are picking or choosing where you're going to rely on scripture as the final or only authority. The Bible is the only authority, so if you're relying on something else, it isn't the only authority anymore. That's the problem.
Old Earth Creationism
The fact that someone says "young earth" before "creationism" reveals the problem here. Creationism is only "young earth" in the Bible. The second book says "old earth," so men attempt to compromise an unbridgeable gap.
Much of so-called Christian psychology places man's observations on par with the Bible. This results in a mixture of humanism (Freudianism, Skinnerism, etc.) and scripture that really don't mix. The Bible says the problem is sin, but with man's discoveries saying something different, a compromise is negotiated.
Church Growth Movement
Pragmatism adds another book to church growth. More is relied upon than biblical methodology because it works better. Paul rejected the extras that were characteristic of the false teachers to attract a crowd (1 Corinthians 1-3). They counted their success as proof of God's work.
The Bible teaches perfect preservation and availability of every Word to churches in general. That's one book. It's the book Christians believed for centuries, so this is also historic Christian, the received text position. Along came textual criticism to add another book. Like evolution was added to young earth and psychoanalysis was added to counseling, both "sciences," the science of textual criticism (and it is again a "science" in the same sense as the other two) was added to the one book as a second book for deciding what God's Words were.
The one book says the gates of hell would not prevail. A second book, the scientific method, was added for deciding whether true churches existed. Out of this comes the English separatist theory, accepted as fact like the theory of evolution is accepted as fact.
Any time you add a second book, you're no longer depending on scripture alone and you get a perversion of biblical and historical doctrine. In each of the above instances, the second book contradicts what the Bible teaches. I believe that more could be added to this list, but this should be a start in consideration.
A certain group of men would reject the second book for young earth creationism. You can add some more for Christian psychology and the church growth movement. The point, it seems, would be a repudiation of the second book in favor of sola scriptura. But the same characters who spurn the first three in this list, accept the last two by relying on the second book. They are no longer sola scriptura. In each case above, some kind of humanism or rationalism, perhaps influenced by scholarship or academia, what might be called intellectual pride, has perverted the truth.
All false religion believes something more or less than the Bible, and often it adds something. Roman Catholicism adds tradition. Mormonism adds the Book of Mormon among others. Charismaticism adds experiences. A two book approach does what these false religions do. Some two book approaches aren't enough to change the gospel....yet. But they are in the trajectory. Any time the second book becomes acceptable, it can be added in other ways or places or positions.
Relying on the second book is in fact walking by sight. Faith is what pleases God (Heb 11:6). We do not please God when we depend on the second book. We are not walking by faith.
Some of you reading, think that old earth creationists should repent. You think that men should turn from the addition of psychology to counseling. It's clear to you. Could you take a moment to consider whether you are doing the same thing that you repudiate elsewhere?