Jude
3Beloved, 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.
4For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the grace of our God into lasciviousness, and denying the only Lord God, and our Lord Jesus Christ.
If we all stay in the word of God, we will be able to spot these men.
The problem is that so many today are so biblically illiterate that it is very easy for false teachers to creep into the church.