What is the faith
Jude speaks of "the faith". He had something specific in mind. The faith that was passed down from the teachings of Jesus and carried on through the work of the apostles. The idea is similar to what the Apostle Paul wrote in Ephesians 4:5, One Lord (true) One faith (true).
Ephesians 1:1The New King James Version
Paul, an apostle of Jesus Christ by the will of God,
To the saints who are in Ephesus, and faithful in Christ Jesus: