One of the key tenets of almost all Protestantism is Sola Scriptura. They believe that all truths are contained in the Bible, and that anything from a source outside the Bible is not part of God's revealed truth. On the other hand, Catholics see Sacred Scripture as one of two ways in which God reveals His truth; the other is Sacred Tradition. Not everything that God wanted us to know was necessarily written down in the Bible; some things He told the apostles and they have handed these things down through their successors and to us through Tradition. It is on the dual basis of Scripture and Tradition that our knowledge of God and His truths is based. This belief also means that Catholics do not see the Bible as the final arbiter of truth. The final authority on truths revealed by God is the Church. It is the Church that interprets and guards the Bible to ensure that whatever is taught and believed is actually what God intended. The Bible is very important, but it is a tool which God has given us to reach the truth. It is one of two main tools, but the Church is the one who teaches us how to use the tool properly so as not to hurt ourselves spiritually.
Not the Bible Alone in the Bible
Nowhere in the Bible does it say that we should trust it completely, and it actually tells us to listen to tradition also. “So then, brethren, stand firm and hold to the traditions which you were taught by us, either by word of mouth or by letter.” - 2 Thessalonians 2:15. St. Paul tells the church he has founded that they should listen to both oral and written tradition. Written tradition is of course what we now know as the Bible, but oral tradition is also important. “First of all you must understand this, that no prophecy of scripture is a matter of one's own interpretation, because no prophecy ever came by the impulse of man, but men moved by the Holy Spirit spoke from God.” - 2 Peter 1: 20 – 21. St. Peter tells his listeners that they should not presume to interpret scripture on their own. Rather, it is the work of the Holy Spirit to tell us what scripture means. Of course it could be argued that the Holy Spirit is in each person who can then decide for himself what God meant in a particular passage. However, even looking at history tells us that this is not the case. There is a continual splintering of Christian churches as different people read scripture in different ways and come to completely opposite conclusions. This cannot be the work of the Holy Spirit, because only one of the conclusions can be the truth, and the Holy Spirit would never lead someone away from the truth. Thus individuals must not have been given the Holy Spirit in the capacity to interpret scripture. This has been left to the Church, to guide all her members to the truth.
Early Church Fathers understanding of place of scripture
The early Church fathers wrote much about scripture and its place in the tradition of the Church and the search for truth.
It is needful also to make use of tradition, for not everything can be gotten from sacred Scripture. The holy apostles handed down some things in the scriptures, other things in tradition. - Epiphanius of Salamis Medicine Chest Against All Heresies 61:6
“Verse 15.So then, brethren, stand fast, and hold the traditions which you were taught, whether by word, or by Epistle of ours.” Hence it is manifest, that they did not deliver all things by Epistle, but many things also unwritten, and in like manner both the one and the other are worthy of credit. Therefore let us think the tradition of the Church also worthy of credit. It is a tradition, seek no farther. Here he shows that there were many who were shaken. - John Chrysostom Commentary (Homily) of 2 Thessalonians 2:15
Of the beliefs and practices whether generally accepted or publicly enjoined which are preserved in the Church some we possess derived from written teaching; others we have received delivered to us in a mystery by the tradition of the apostles; and both of these in relation to true religion have the same force. And these no one will gainsay—no one, at all events, who is even moderately versed in the institutions of the Church. For were we to attempt to reject such customs as have no written authority, on the ground that the importance they possess is small, we should unintentionally injure the Gospel in its very vitals; or, rather, should make our public definition a mere phrase and nothing more. - Basil the Great On the Holy Spirit, 27
Seeing there are many who think they hold the opinions of Christ, and yet some of these think differently from their predecessors, yet as the teaching of the Church, transmitted in orderly succession from the Apostles, and remaining in the churches to the present day, is still preserved, that alone is to be accepted as truth which differs in no respect from ecclesiastical and apostolic tradition. - Origen On First Principles Bk. 1 Preface 2
These first century authors demonstrate their belief that the Bible alone is not sufficient. Oral Tradition is also a part of the deposit of faith left by Jesus, and without the Church, we cannot be assured of gaining any part of the truth strictly by reading the scriptures.