We sometimes tend to think of the early church as pristine, pure, and untroubled by serious error. The truth is, it wasn’t always that way at all. From the the very beginning, the enemies of truth launched an effort to infiltrate and confuse the people of God by mangling the truth and by blending lies with Christian doctrine. Attacks against the truth regularly came not only from persecutors on the outside but also from false teachers and professing believers within the visible community of the church. . . .  So false teachers had already infiltrated the [first century] church. They were at that moment safely ensconced in the community of believers. They were being accepted as fellow believers, and their poisonous false teaching was spreading in this church. Jude urged believers to oppose them rather than embrace them. The life of the church depended on it.

– John MacArthur