Beloved, while I was very diligent to write to you concerning our common salvation, I found it necessary to write to you exhorting you to contend earnestly for the faith which was once for all delivered to the saints. For certain men have crept in unnoticed, who long ago were marked out for this condemnation, ungodly men, who turn the grace our God into a license immorality and deny Jesus Christ, our only sovereign and Lord (Jude 3-4).
Jude could not resist appealing to his readers (us today) that we should contend earnestly for the faith. This powerful expression contend earnestly stresses the need to the defend the truth continually and vigorously (1at Tim. 1: 18).
In referring to the faith, Jude speaks of the faith which constitutes the Christian faith, the faith of the gospel. God's objective truth.
Luke wrote about this I Acts 2:42, noting that the early believers continually devote themselves to the apostles teaching(1st Cor.15:1-4; 2nd Thess.3:6). Paul admonished Timothy and to us today to protect that faith
Timothy 1:13-15
King James Version
13 Hold fast the form of sound words, which thou hast heard of me, in faith and love which is in Christ Jesus.
14 That good thing which was committed unto thee keep by the Holy Ghost which dwelleth in us.
15 This thou knowest, that all they which are in Asia be turned away from me; of whom are Phygellus and Hermogenes.