Jude 1:4

For certain people have crept in unnoticed. The ungodly person whose condemnation was predicted long ago, for they distort the grace of our God into decadence and immoral freedom (viewing it as an opportunity to do whatever they want), and deny and disown our only Master and Lord, Jesus Christ.

As in the time of Jude and now, our faith has been challenged by outside influences.  The devil doesn’t want us to be Christians and believe in Jesus or the Bible. That is why Jude tells people to guard their faith and not be drawn away from what they have been taught. Just as we must keep our guards up and focus intensely and the word in our hearts.