Deposit of Faith

Faith is necessary for salvation.

For God so loved the world that he gave his only Son, so that everyone who believes in him might not perish but might have eternal life.  
John 3:16 

God has revealed himself to us so that we might have faith in Jesus Christ and the One who sent him. It is through God’s grace that we have the gift of faith. The Deposit of Faith contained in Sacred Scripture and Sacred Tradition nurtures our growth in faith and teaches us how to live as Christ’s disciples.
-The Catholic Faith Handbook

What is the Deposit of Faith?
 

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

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

"Behold, I am with you always, until the end of the age."
Matthew 28:20

God is the only source of Revelation. The Deposit of Faith is the truth of God's Revelation as expressed in Sacred Scripture, which is the inspired and written Word of God, and Sacred Tradition, which is the Word of God as taught and transmitted through the teaching authority of the Church. 

There exists a close connection and communication between Sacred Tradition and Sacred Scripture. For both of them, flowing from the same divine wellspring, in a certain way merge into a unity and tend toward the same end. (DV 9; cf. CCC 86, 97)

Sacred Scripture is the Word of God, written under the inspiration of the Holy Spirit, contained in the collection of sacred books that have God as their true author and are entrusted to the Church (cf. DS 3006).
Sacred Tradition from the Latin traditio, meaning "to hand on") is the Word of God as received from Christ himself through the Apostles and transmitted to us without aIteration—as it were, from hand to hand—by the Church with the assistance of the Holy Spirit (cf. DS 1501; DV 9). Sacred Tradition must not be confused with "traditions" of a pious, devotional, theological, or disciplinary nature. (Cf. CCC 76-78, 81-83,2033)

Both Sacred Scripture and Sacred Tradition are to be regarded with equal reverence and devotion. Sacred Tradition preceded the New Testament writings; the first Christians had no written Gospels and only the preaching of the Apostles, the oral tradition, to instruct them about Christ and his teachings. Through Tradition we know which books are inspired by the Holy Spirit, that is, the list, or canon, of the books that make up Holy Scripture. (Cf. CCC 83)

Testimonies of Tradition date back to the first centuries and have been preserved in either ancient liturgical or disciplinary texts and practices or the writings of early Christian authors. (Cf. CCC 120,175)

-The Didache Bible