Jude identifies himself as the servant of Jesus and the brother of James.  He addresses his audience as "the called, the beloved in God the Father, and kept for Jesus Christ."   
To those of us in this abiding relationship with our Lord, Jude prays for multiplied mercy, peace, and love.  What a precious prayer of blessing!

Verse 3 strikes me today in a way that makes me perk up my ears... Jude says he really was planning on writing about "our common salvation"... but instead, " felt the necessity to write you appealing that you contend earnestly for the faith.."  God changed the direction of Jude's letter.  It takes on a more serious tone.. a urgent message, a life or death warning.  One that is just as important to us today as it was for the readers back then.

"Contend earnestly for the faith".   Fight the good fight...
For an enemy has come in:
They are ungodly people who creep in unnoticed.
They turn grace into "licentiousness"... into filth.
They deny Christ. ( v4)
They "defile the flesh";
They "reject authority";
They "revile angelic majesties." (v8)
They are ungodly people who do ungodly things in an ungodly way. (v15)
They are grumblers.
They are fault finders.
They follow their own lusts.
They speak arrogantly. ( v16)

Remember, Jude teaches, that the Lord is Judge.
He brought the people of Israel out of Egypt, but destroyed the unbelievers in the wilderness. (v5)
He removed the unfaithful angels from glory and "has kept [them] in eternal bonds under darkness for the judgement of the great day."
He made Sodom and Gomorrah an example of judgement when they "indulged in gross immorality.."

He will execute judgement upon the ungodly people who have come in like "hidden reefs".
They take part in our communion fearlessly, though they are full of wickedness.
Jude calls them:
"clouds without water, carried along by winds; autumn trees without fruit, doubly dead, uprooted; wild waves of the sea, casting up their own shame like foam; wandering stars for who the black darkness has been reserved forever." (v12-13)
They are mockers.
They are dividers.
They are worldly.
They are "void of the Spirit" (v19)

These are who we must fight against, not in our own power.. but like Michael, the archangel, we say, "The Lord rebuke you.". 
In these "last days" we must be " building yourselves up on the most holy faith; praying in the Holy Spirit; keeping yourselves in the love of God, waiting anxiously for the mercy of our Lord Jesus Christ to eternal life."  

And as we wait, we minister.
We have mercy on those who doubt.
We snatch the lost out of the fire when we can.
We fearfully and mercifully help those who are so polluted with sin that we dare not even touch them.
For we walk in faith that He.. "is able to keep you from stumbling, and to make you stand in the presence of His glory blameless with great joy."

".. to the only God our Savior through Jesus Christ our Lord, be glory, majesty, dominion and authority, before all time and now and forever. Amen".