Sacred Scripture is the written testimony of the divine Word, the canonical memory that attests to the event of Revelation. However, the Word of God precedes the Bible and surpasses it. That is why the centre of our faith isn’t just a book, but a salvation history and above all a person, Jesus Christ, the Word of God made flesh.Pope Francis, Address to Pontifical Biblical Commission
Unlike Protestants, we as Catholics do not believe in the Bible alone (sola scriptura), but that God’s Word to us is contained in both Scripture and Sacred Tradition. We believe that the Divine Revelation that God gave us is a single, consistent deposit of faith, but that this one deposit is communicated to us through both the written form and the oral Tradition. This oral Tradition has been faithfully passed down in the Catholic Church from the Apostles, who received it from Christ Himself as well as through the light of the Holy Spirit.
Before I lay out the reasons Catholics believe in Sacred Tradition, I would like to note that when I refer to the Sacred Tradition of the Church, this is different from what I will refer to as “traditions” or pastoral practices. Sacred Tradition only refers to those fundamental doctrines on faith and morals, which have been passed down unchanged from the Apostles. On the other hand, traditions with a lowercase ‘t’ refers to those pastoral practices which can change over time [and should always be in accordance with the faith and morals of the Church], depending on the current culture and needs of the Church. An example of a doctrine followed according to Tradition (and Scripture) is the belief in the Real Presence of Christ in the Eucharist. An example of a teaching on morals is the indissolubility of marriage. Examples of traditions, not from infallible Tradition, are the practice of abstaining from meat on Fridays and of which direction the Priest faces during Mass.
It is reasonable and necessary to follow this Sacred Tradition (uppercase ‘T’) for several reasons:
1. The Bible Says So
First of all, nowhere in Scripture does it clearly state that one should believe in the Bible alone. And more importantly, in many instances it implies the opposite. Here are several examples:
In the Gospel of John, the Apostle demonstrates that not everything Christ said and did is written down in Scripture:
There are also many other things that Jesus did, but if these were to be described individually, I do not think the whole world would contain the books that would be written.John 21:25
Christ’s command to the Apostles was not to write, but to preach and to proclaim—“Go into the whole world and proclaim the gospel [good news] to every creature” (Mark 16:15). This preaching includes, but is not limited to, writing; in fact most of the Apostles and their disciples’ lives after Christ’s death was dedicated to going about the world and speaking to all nations about what Christ taught them (in fact, this was before the New Testament was even compiled; see reason #2).
St. Paul says that his oral teaching is also the word of God:
And for this reason we too give thanks to God unceasingly, that, in receiving the word of God from hearing us, you received not a human word but, as it truly is, the word of God, which is now at work in you who believe.1 Thessalonians 2:13
And again in 2 Thessalonians, Paul acknowledges the oral tradition as of equal value to the written:
Therefore, brothers, stand firm and hold fast to the traditions that you were taught, either by an oral statement or by a letter of ours.2 Thessalonians 2:15
In 2 Timothy, Paul also says that this oral teaching is to be faithfully passed down, specifically by those whom he has “laid his hands upon,” e.i. passed on the gift of the Holy Spirit as in Holy Orders (the passing on of the priesthood):
For this reason, I remind you to stir into flame the gift of God that you have through the imposition of my hands. … Take as your norm the sound words that you heard from me, in the faith and love that are in Christ Jesus. Guard this rich trust with the help of the holy Spirit that dwells within us.2 Timothy 1:6, 13-14
The imposition of hands here refers to the passing on of the Holy Spirit in a special way to priests, so that they may live out their calling to shepherd the faithful and to “guard the rich trust” of the faith as it has been passed down from the Apostles. Thus, this also implies that “the effective transmission of the apostolic tradition is not simply a matter of teaching, but involves a sacramental charism given by the Spirit through Holy Orders.”
2. The Jews and Early Christians Passed on Teachings Orally
Before the writing and compiling of the New Testament, the early Christians had to pass down the faith orally, as did the Jews before and even after the writing down of the Old Testament. Even as the Scriptures were written down, they continued to pass on teachings orally as well. During the times of the early Christians and before them, it was very common to pass on teachings (as well as memorized scriptures) orally. This was especially important because many were not literate; and if one must be able to read the Bible to be saved, this implies that literacy is a requirement for salvation—which is obviously absurd. Even today, there are those, especially in poor communities, who cannot read and do not even have anyone who can read the Scriptures to them.
3. Tradition Tells Us the Bible is Inspired
More importantly, if we are to rely on Scripture alone, then how do we know it is inspired by the Holy Spirit in the first place—by taking its own word for it? No, we trust the oral Tradition that has been passed down which says that the Bible is inspired. All Christians follow this oral tradition that says the Bible is the Word of God, even if they do not recognize it as a tradition. Furthermore, no part of Scripture tells us which writings are inspired or not—it is Tradition that determined which were truly in accordance with Christ’s teaching and which were not. Thus, those who deny Sacred Tradition and follow the rule of private interpretation not only have the freedom to interpret Scripture however they want–they have the freedom to affirm or deny which books are inspired Scripture and which are not, purely based on their own opinions. Howevever, the Catholic Church affirms Scripture as coming from and being a part of Sacred Tradition. This Tradition never contradicts anything in Scriptures, nor adds to the teachings given to us by Christ, but rather explains what is taught explicitly and implicitly in Scripture, always in coherence with what Christ and the Holy Spirit taught to the Apostles.
4. We Need Tradition to Interpret Scripture
Furthermore: Even after the official canon (list) of the Scriptures was determined, we need Sacred Tradition in order to know how to properly interpret Scripture. No one can deny that the Bible is a difficult book to understand—it is why we have Biblical scholars who study for years to try to understand it properly. We may ask the Holy Spirit to guide us, but we can see through experience that just because we have a genuine desire to understand the truth and have a relationship with God, does not mean that our personal interpretation of the Scriptures will be correct. If that were true, we wouldn’t have so many different denominations (most of whose members are probably genuine) disagreeing on so many different issues. This is the problem with sola scriptura and the rule of private interpretation. Christians are more divided than ever because they don’t have (or rather, don’t acknowledge) an authoritative interpreter. The Catholic Church is this authoritative voice that Christ knew we needed (which leads to reason #5).
5. God Promised to Protect His Church
God, being all-knowing, would know that over the centuries many would be led astray from His true teaching; thus, it seems necessary that, being an all-knowing and all-loving God who has done everything possible to bring us to salvation (including dying on the cross), He would want to provide a way for His Church to remain in the fullness of the truth. And this is exactly what He did—Christ promised that He would be with His Church until the end of time:“Behold, I am with you always, until the end of the age” (Matthew 28:20). And He promised to Peter (the first Pope) that “the gates of the netherworld [hell, the devil and his lies, including error] shall not prevail against it” (Matthew 16:18).
God has fulfilled His promise by protecting the Church’s dogma from error (though statements of the Church have varying degrees of certainty according to the level of authority with which they are made: see Degrees of Church Authority: Should We Believe Everything the Catholic Church Says?). This is simply a miraculous and special grace of the holy spirit–though the faithfulness of the Church’s teachings can be confirmed by examining what the early Christians really believed (see #7).
6. The Early Church Fathers Trusted Sacred Tradition
This oral Tradition was trusted by the early Church Fathers. For example, St. Irenaeus, who was bishop of Lyons during the second and third centuries, says that he had listened to and remembered very carefully the things which St. Polycarp, martyr and disciple of St. John the Apostle, had told him. (Polycarp was also taught by St. Ignatius of Antioch, who was a disciple of St. Peter and St. John). Irenaeus writes:
“I can describe the place where blessed Polycarp sat and talked …. I remember how he spoke of his intercourse with John and with the others who had seen the Lord, how he repeated their words from memory, and how the things that he had heard them say about the Lord, His miracles and His teachings, things that he had heard direct from the eye-witnesses of the Word of Life, were proclaimed by Polycarp in complete harmony with Scripture. To these things I listened eagerly at that time, by the mercy of God shown to me, not committing them to writing but learning them by heart. By God’s grace, I constantly and conscientiously ruminate on them.”St. Irenaeus
Thus Irenaeus, in his role as Bishop, fully accepted his duty to know and pass on not only the Scriptures, but also the Word of God spoken to Him by those who had learned this from the Apostles (who in turn had learned this from Christ).
7. The Early Christians Believed What the Catholic Church Believes
The only other way to confirm that the Catholic Church’s Tradition has remained true to the teachings of Christ is to examine what the Christians of the Early Church really believed—and those who have done so have found that they were overwhelmingly in accordance with the doctrines of the Catholic Church (for example: The Real Presence of the Eucharist in the Early Church). Other examples would be topics for another day.
May God bless you and keep you!
 Lawrence Feingold. Faith Comes from What is Heard: An Introduction to Fundamental Theology. 199.
 From Eusebius, Historia eccliesiastica, 5.20.6, in which Eusebius quotes a letter from St. Irenaeus.