Sacred Scripture itself teaches that all is not written it its books and that there are some Truths that the Spirit will continue teaching us through His Church:
“Now there are also many other things that Jesus did, which, if each of these were written down, the world itself, I suppose, would not be able to contain the books that would be written.” (John 21:25).
Therefore, there are things that Jesus did which are not written in the Bible but they are contained in Sacred Tradition.
“I still have many things to say to you, but you are not able to bear them now. But when the Spirit of truth has arrived, he will teach the whole truth to you. For he will not be speaking from himself. Instead, whatever he will hear, he will speak. And he will announce to you the things that are to come.” (John 16:12-13). There are things that Jesus did not say to the Apostles while He was here on earth, but He promised that the Holy Spirit will continue to guide them and, of course, their successors for He promised that He will be with them until the end of age (Matt 28:20), that is until He comes back in glory to judge the living and the dead (Matt 13:39-40) (13:49) (Matt 24).
“I have much more to write to you, but I am not willing to do so through paper and ink. For I hope that I may be with you in the future, and that I may speak face to face, so that your joy may be full.” (2 John 1:12)
“I had many things to write to you, but I am not willing, through ink and pen, to write to you.” (3 John 1:13).
Jesus entrusted His authority to the Apostles: “So Jesus said to them again, “Peace be with you; as the Father has sent Me, I also send you.” (John 20:21).
“So then, brothers and sisters, stand firm and hold fast to the teachings we passed on to you, whether by word of mouth or by letter.” (2 Thes 2:15)
“if I am delayed, you will know how people ought to conduct themselves in God's household, which is the Church of the living God, the pillar and foundation of the truth.” (1 Timothy 3:15).
Thus, the Bible itself teaches in its own writings, that it is not the sole source if instruction, guiding and teaching.
The fact that God has selected some men for the particular task of speaking for Him, or to represent Him is evident through the history of salvation. We see it in the case of Moses when our Lord tells him:
“Speak to him [Aaron], and put my words in his mouth. And I will be in your mouth and in his mouth, and I will reveal to you what you must do. He will speak for you to the people, and he will be your mouth. But you will be with him in those things that pertain to God.” – (Exodus 4:15-16)
“And the Lord said to Moses: “Behold, I have appointed you as the god of Pharaoh. And Aaron, your brother, will be your prophet.” (Exodus 7:1)
Words like “I will be in your mouth and in his mouth” or “I have appointed you as the god of Pharaoh” refers to the authority that Moses has received from God Himself in order to speak infallibly at those particular instances (for Moses is not infallible himself) for God. God appointed Moses to be his vicar, to speak in God’s place.
Then thousands of years later when Jesus came to this world, He respected the authority of those who taught from the “chair of Moses”, even though those who were in the “chair”, that is, in his position, were sinners and showed bad examples.
“Then Jesus spoke to the crowds, and to his disciples, saying: “The scribes and the Pharisees have sat down in the chair of Moses. Therefore, all things whatsoever that they shall say to you, observe and do. Yet truly, do not choose to act according to their works. For they say, but they do not do. For they bind up heavy and unbearable burdens, and they impose them on men’s shoulders. But they are not willing to move them with even a finger of their own.” (Matt 23:1-4).
Jesus taught us to obey those who are in the authoritative chair of Moses, even though they showed bad example to the people. He said not to follow their own particular example because they did not follow what they themselves taught with sincerity of heart. In other words, they only wanted to be seen and acclaimed by men. Even so, Jesus taught to obey them because of the position they were at, the “chair of Moses”.
John 3:27; John 15:5; Matthew 19:26