Those in spiritual leadership must take care to contend for the faith, that is, to teach the truth accurately and oppose error. Why is this so important?
My people are destroyed for lack of knowledge;
because you have rejected knowledge,
I reject you from being a priest to me.
And since you have forgotten the law of your God,
I also will forget your children. Hosea 4:6
Beloved, although I was very eager to write to you about our common salvation, I found it necessary to write appealing to you to contend for the faith that was once for all delivered to the saints. For certain people have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ. Jude 1:3-4
In both readings today, we see God indicting those who have forsaken the truth and who misled those who looked to them for guidance.
Hosea, like other prophets, had a message of warning and hope for Israel and Judah. The Lord called him to depict God’s mercy and grace toward His faithless people by taking a prostitute for his wife. God told them they were destroyed for lack of knowledge. Their teachers taught lies rather than God’s law. The priests had facilitated national sin.
In a similar way, God called believers in Jude’s day to contend for the faith, that is, the doctrine He gave the Church through the Apostles and Prophets. False and wicked teachers attacked this truth in their deceitful and treacherous ways. They misused God’s grace as an excuse for sensuality. They denied the Lord Jesus Christ. Among other vices, they relied on their dreams. They claimed to get their own truth by direct revelation, a practice Paul also condemned (Colossians 2:18). 
Think about it
Truth matters. It matters what we believe, and, if we are in the position of teachers, it matters to all whom we influence for good or bad. Be sure you know the truth of God’s word and that those you learn from contend for the faith and are not relying on the inventions of their minds.
 The Reformation Study Bible, Sanford, FL, Reformation Trust, 2015, p. 2292. See note on verse 8