"Beloved, when I gave all diligence to write unto you of the common salvation, it was needful for me to write unto you, and exhort you that ye should earnestly contend for the faith which was once delivered unto the saints." (Jude 3)
Jude long ago addressed a problem in his day which is still very real in our day among Christians. It is easier and more comfortable just to teach and preach about the blessings of our common salvation than it is to contend for the faith, but the latter is more "needful." The word conveys the idea that he was so constrained, evidently by the Holy Spirit, as actually to be in distress about this compelling need. Similarly, his exhortation to "earnestly contend" does not mean to "be argumentative," but rather, to "agonize with intense determination." It is one word in the Greek, epagonizomai (literally, "agonize over"). Defending and contending for the faith is serious, urgent business.
That which we are to defend is "the faith"--the whole body of Christian truth, wherever it is under attack. It would, of course, be especially important to contend for the doctrine of special creation, which is the foundation of all others, and which is the doctrine perpetually under the most concerted and persistent attack by the adversary.
That faith has been, long ago, "once delivered" to the saints. The sense of these words is "once for all turned over for safekeeping." The Lord has entrusted us with His Word, completed and inscripturated, and we must keep it, uncorrupted and intact, for every generation until He returns, preaching and teaching all of it to every creature, to the greatest extent we possibly can.
Finally, note that the safeguarding of the faith was not merely to specially trained theologians or other professionals, but to "the saints." Every Christian believer is commanded to "earnestly contend for the faith." HMM