The metaphor I turn to most often when talking about Scripture and Tradition is that of little league baseball. Imagine if I decided to coach a little league team and, deciding that my "coaching traditions" would only corrupt the pure game of baseball, I gave each of these little sluggers a copy of the official book of baseball rules and said, "Have at it, boys. Memorize it. Highlight it. Our first game's in three weeks."
That first game would be a disaster. The book of rules contains all the rules and protocol to play the game of baseball, but there's something missing when we remove the good coach, who brings with him a tradition of good coaches right on down through the history of the game. The book of rules tells us that we must swing the bat and hit the ball, but it doesn't show us how to choke up, to keep our eye on the ball, or to follow through.
Likewise, many Christian groups try to go by the book of rules alone, defining their Christianity by the Bible alone (sola-fide in Latin). This is a mistake. A big one.
As Catholics, we believe that the Bible, like that book of baseball rules, is "materially sufficient", in that it contains, either explicitly or implicitly, all that has been revealed about salvation. Scripture is not, though, "formally sufficient", in that we can interpret Scripture by the plain sense of its words.
And guess what ... Scripture agrees. Nowhere does Scripture ever tell us to go by Scripture alone. Think about this. If we are to "speak where Scripture speaks and be silent where Scripture is silent", as so many claim, where does Scripture "speak" about going by the Bible alone? Jesus never wrote any of his teachings down (at least in a form that is available to us today), nor did he ever command his apostles to put everything in print for his Bible-alone Christianity. The inspired writers also never tells us that Scripture should be our final authority. In fact John even goes so far in his epistles as to tell us that he would rather pass his message "face-to-face", rather than using "paper and ink" (2 John 12).
Not only does Scripture not tell us to go by Scripture alone, but there are many places where we are told to go by the oral teachings of Christianity, as well, such as 2 Tim. 2:2 and 2 Thes. 2:15.
Why? Well, to use another baseball analogy, I once played with a woman who had an artificial eye. Because she had vision in only one eye, she had lost her depth-perception, which is given through our two-perspective vision. Playing outfield was nearly impossible for her because she would very often misjudge pop-flies.
Likewise, combining Scripture and Tradition gives us depth perception, and to see the danger of limiting ourselves to only one eye (Scripture alone), consider how horribly Christianity has splintered since the reformation. Some sources put the number of distinct Christian groups at tens of thousands. And many of these groups, which disagree with one another over key doctrine, claim to go by the "simply truth" of Scripture alone.
Christ prayed for unity within his church (John 17), and Paul makes clear that this is meant to be a doctrinal unity (1 Cor. 1:10; Phil. 2:2). Would Christ be happy to find up to three different Christian churches facing each other on a street corner, each preaching a different version of his gospel? Would his prayer be answered by the hundreds of churches listed in any given yellow pages section of the phone book?
Maybe someday all Christians will recognize the unity possible through Scripture and Sacred Tradition, both preserved through the "good coach" of the Catholic Church (who is protected from error by the Holy Spirit).
Unfortunately, many today still follow the man-made doctrine of "Scripture-alone", which robs a Christian of his spiritual depth-perception. And leaves him out in right field, missing the ball almost every time.