God said, (my bolding added)
Matt 28:19-20
Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen.
And they did:
2 Cor 4:6 For God, who commanded the light to shine out of darkness, hath shined in our hearts, to give the light of the knowledge of the glory of God in the face of Jesus Christ.
These teachings from the Apostles are the foundation for the entire body of teachings passed down to us, which includes not just sacred scripture but all the "things" He had taught them. The teaching authority Christ gave His Apostles, they passed on to Bishops who took their place. And that process can be shown again by Saint Paul:
2 Tim 2:2 And the things that thou hast heard of me among many witnesses, the same commit thou to faithful men, who shall be able to teach others also.
It is that transmission of knowledge from Christ through the Apostles is passed down to us and that we call Tradition, and it is distinct from sacred scripture. The "T" is capital to distinguish it from tradition/legend which is what we were talking about with the assumption of the Mother of God. Because of the source (Christ Himself) both sacred scripture and this Tradition are considered the Word of God.
The Holy Fathers have expounded upon this Apostolic Tradition and we have their writings (and lives) as first hand account of how this Tradition has continued to bless the Church and our lives for it. This Apostolic Tradition stands with sacred scripture, not against. Together with the teaching authority (Magisterium) Christ gave the Church (bind and loose - Matt 16:19, 18:18), these three (Sacred Scripture, Tradition and Magisterium, work together to preserve and pass down for all mankind all that we are to believe (all that God has revealed to mankind). The Church preserves that knowledge, what we are to believe, and that only possible by His Grace, the Holy Spirit (comfort and guide) and His promise:Matt 28:20
"Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen."