Sacred Scripture is just another way of saying “the Bible.” Sacred Tradition refers to what is not written but is as much divinely revealed as Sacred Scripture.
The best way to understand the Catholic concept of revelation is to see it as being the revealed Word of God. Sacred Scripture is the written Word and Sacred Tradition is the unwritten or spoken Word. Both the written and the unwritten Word make up the totality of divine revelation.
This does not mean that scripture and tradition are in competition with one another. In fact, since they both come from the same source (God), neither can contradict the other. Sacred Tradition predates Sacred Scripture, since the unwritten Word was spoken for centuries and generations before it was ever written down. Hebrew children were verbally told about Adam and Eve, Cain and Abel, Noah and the Ark, Abraham and Isaac, etc., from their parents and grandparents. There were no books or bibles in antiquity.
Moses may have been the first person to write the book of Genesis, but its content was known centuries before he was ever born. People spoke the revealed words of the Old Testament; only later were they written by the sacred author.
Even in the New Testament, Sacred Tradition predates the written Gospel. Jesus first said and did what He did in time, then His apostles told people (preached). Only later did the Evangelist (Gospel writer) write down the text of the Gospel. Matthew, Mark, Luke, and John did not take notes while Jesus preached or performed miracles. The unwritten word came first and was followed by the written word; both contain the same message.
Even the Gospel of John ends with, “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” (21:25). Those other things are part of Sacred Tradition. That Jesus never married is not explicitly stated in Sacred Scripture, but wherever the Bible is silent or ambiguous, Sacred Tradition fills in the gap. Sacred Tradition is that Jesus never married nor had any children. Sacred Tradition is that the Virgin Mary remained a virgin her entire life and had no other children other than her son Jesus. (For more on this, see Question 31.) Sacred Tradition determined what books belong in the Bible and which ones do not.
Rather than a dichotomy between Sacred Scripture (Bible) and Sacred Tradition, both are like two lungs in one body, or two sides of one coin. Both originate from God and both are authentically interpreted by the Church.
Sacred Tradition is equal to Sacred Scripture since both originate with God Himself. Divine revelation comes from both vehicles of the one source (God). If there appears to be a contradiction between the two, then the mistake is in the copy, translation, or interpretation of the written text. Since the Bible in and of itself never gives an exhaustive list of which books should or should not be considered part of the Bible, only Sacred Tradition can tell infallibly which books are inspired and which are not. No inspired book says “I’m inspired,” and no sacred author was aware that he was being inspired since none ever say so.
Sacred Tradition is different from human tradition. Tradition comes from the Latin word artumon, meaning “to hand down.” Human traditions are imperfect, flawed, fallible, and open to change. Many of the religious customs of both Jews and Christians were human tradition, that is, man-made. Sacred Tradition originates from God and is divinely made. It cannot change. That is why there will never be any more books added to the Bible. Sacred tradition says that revelation ended with the death of Saint John the Beloved (around 100 AD). Any manuscript discovered afterward will never be included in future versions of the Bible. Apocryphal Gnostic writings like the Gospel of Mary Magdalene, for example, were written in the third or fourth century AD.
There is a distinction between what the Church considers Sacred Tradition and what are merely human traditions. Saint Paul says in 2 Thessalonians 2:15 (from the King James Version), “Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle.” The original Greek word used by Paul, paradoseis, means “traditions.”
This does not contradict what he says in 2 Timothy 3:16: “All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness,” since he never says Scripture alone (sola scriptura) or only in Scripture is found inspiration, as was held by Martin Luther. It is the same Paul who authored 2 Thessalonians 2:15 and 2 Timothy 3:16, and both epistles are in the Bible and considered inspired text. He also says in 1 Corinthians 11:2, “I commend you because you remember me in everything and maintain the traditions even as I have delivered them to you.”
Saint Paul, from what is quoted in the previous paragraph, has no problems or issues with Sacred Tradition. He is concerned about human traditions and customs which have been taken out of context or given more attention and priority than those of divine origin. Matthew 15:3 has Jesus rebuke the Pharisees: “And why do you transgress the commandment of God for the sake of your tradition?”
The key here is the word “your.” When Jesus speaks of “your tradition,” it is distinct and separate from “divine” or “sacred” tradition. The man-made, human traditions the Pharisees scrupulously maintained were not superior or even equal to the traditions which originated from God.