Earnestly Contending for the Faith

In these two verses Judes tell us why he wrote this letter: a great problem had arisen in some of the churches. Some infiltrators had crept in and were teaching false doctrines, seeking to undermine the true doctrines of the once-for-all-delivered-faith, as well as seduce believers into imitating their immoral behavior.
In this passage Jude reminds his readers of the common gospel confession that must be agonizingly protected and warms them of the spiritually mortal danger from subversive underminers. It was a crucial message back then and is the same for us today.