Jude 3,4

"Beloved, while I was making every effort to write to you about our common salvation, I felt the necessity to write you appealing that you contend earnestly for the faith which was once for all delivered to the saints.  For certain persons have crept in unnoticed, those who were long beforehand marked out for this condemnation, ungodly persons who turn the grace of our God into licentiousness and deny our only Master and Lord, Jesus Christ."

With apostasy as its one theme throughout, the book of Jude is unique in the New Testament.  Other writers make reference to it, of course, but only Jude devotes his entire letter to this important issue.  Interestingly, his initial intention was to focus upon the salvation which is common to all the elect.  Yet, the Holy Spirit impresses upon him the urgent need to, instead, exhort his readers to beware of apostates who have infiltrated their fellowships.
In so doing, Jude calls upon them to (a) know the faith, (b) exercise discernment, and (c) earnestly confront error.

(a)  People act on what they believe... making sound doctrine of pivotal importance for godly living and acceptable worship of the living God.  We either embrace and act upon revelation (God's Word) or speculation (our own ideas of what is right, wrong, and what God is like).
Jude's exhortation centers upon "the faith which was once for all delivered to the saints."  The faith for which we are to contend earnestly is the body of truth known as the Scriptures.  The sixty-six books are the completed and final revelation of God to His people, and the only Book containing God's promise of His infallible inspiration (II Timothy 3:16).  The more diligently God's people know His Word, the better able they will be to earnestly contend for the truth to an unbelieving world.  

(b)  One of the greatest needs of the church today is discernment.  As evidenced by the multitude of cults, apostate denominations, and sects, it is obvious that the enemy has been adroitly and insidiously planting tares among the wheat for centuries.  At the same time he has cleverly used the infiltration of false brethren and teachers into the visible church, who have cast doubt upon the inspiration and literal interpretation of the Scriptures.  Forsaking the divine authority of the Bible has contributed immeasurably to the decline in discernment, as well as spiritual-mindedness of the people of God.  In our "information age," with its placing of a premium upon education and intellect, the church has by-and-large been intimidated into worldliness and conformity to the assumptions of a godless culture.  An authoritative voice which evaluates society by God's Word has been largely lost, silenced into compromise and cowardice.  Without determined commitment to the Bible as the Word of the living God, as well as unwillingness to submit to the Holy Spirit's operations, many of God's people move through their days blissfully ignorant of the continuous spiritual warfare raging on every hand.  

(c)  Consequently, there is relatively little interest nor intention to confront doctrinal error by believers today.  "Live and let live" has become the mantra of this culture, and the church has absorbed the perspective into its very pores.  "Who are we to judge?", and "Judge not, lest ye be judged," have been so widely communicated from world to church that we seldom search the Scriptures in context to discover the fallacy of such sentiments.  Jude would call us to take God's truth seriously, both personally and corporately, and to love the lost enough to confront them with it.  Truth is not popular today (and actually never has been), but we are called and commanded to speak it in love, consistently, courageously, with full conviction of the Holy Spirit.  May He grant grace to increase our willingness and obedience!