Judah, one of Jesus' half brothers, wrote the book of Jude to Messianic believers who were familiar with Old Testament scriptures and Jewish literature. He uses these texts to refute corrupt teachers who lived immoral lives.
Judah illustrate God's judgment on rebellion, but he warns against rebels corrupting other people. He then challenges the church to contend for the faith and stay faithful to God. As God's new temple, believers must build their lives on the core message of Jesus and then pray, love and obey.
Their lives must reflect their faith because God's grace through Jesus demands a whole-life response that includes moral living. The way they live is the most reliable indicator of what they actually believe.