In today's first reading from 2 Thessalonians 2, St. Paul says, "Stand firm and hold fast to the traditions that you were taught, either by an oral statement or by a letter of ours.
Catholics believe that divine revelation has been handed on to the Church through Sacred Scripture and Sacred Tradition. Many non-Catholics believe in the doctrine of Sola Scriptura, Latin for "Scripture alone." They say that the Bible alone is the sole rule of faith, and they eschew "tradition." As Catholics, we go along with the writings of St. Paul.
Sacred, or Apostolic Tradition (notice the capital “T”) is the oral teachings of Christ given to the Apostles. The first generation of Christians had no written New Testament. They learned the faith through Apostolic Tradition. Remember, the New Testament was not even codified until the end of the 4th century. The Apostles handed on their teachings to their successors - in letters and in oral statements (see today's first reading) - which has continued for 2000 years, to this very day.
Father, we thank you for revealing all truth to your Church through Sacred Scripture and Sacred Tradition. May we always remain faithful to her teachings. Amen.