13-15 Prepare your minds for action and exercise self-control. Put all your hope in the gracious salvation that will come to you when Jesus Christ is revealed to the world. Live as God’s obedient children. Don’t slip back into your old ways of living to satisfy your own desires. You didn’t know any better then. But now you must be holy in everything you do, just as God who chose you is holy. 16 For the Scriptures say, “You must be holy because I am holy.”

17 And remember that the heavenly Father to whom you pray has no favourites. He will judge or reward you according to what you do. So you must live in reverent fear of him during your time here as “temporary residents.” 

18-19  For you know that God paid a ransom to save you from the empty life you inherited from your ancestors. And it was not paid with mere gold or silver, which lose their value. It was the precious blood of Christ, the sinless, spotless Lamb of God.  20-21 God chose him as your ransom long before the world began, but now in these last days he has been revealed for your sake. Through Christ you have come to trust in God. And you have placed your faith and hope in God because he raised Christ from the dead and gave him great glory. 1 Peter 1:13-21 New Living Translation paraphrase (English Standard Version link)

In vs 13-15 Peter tells Christians to turn from an attitude of 'let's live for the day' (which usually results in wrong choices based on short term pleasures) and to fix their minds on living in the light of the coming day of Jesus' return. He gives three excellent reasons why that is important. What's the first reason in vs 16?

He was quoting one of the many times that God said those words through Moses to Israel. For example, in Leviticus 20:26 God says: “You must be holy because I, the Lord, am holy. I have set you apart from all other people to be my very own.” God commands His people to come out of the sinful lifestyle that the world runs after. What’s the second excellent reason Peter gives (in vs 17) for turning from sinful ways?

A literal translation of that verse says that God “judges impartially according to each one’s work”, and therefore we should “conduct ourselves in [reverent] fear [of Him], and with profound respect for Him throughout the time of our stay on earth.” (Amplified Bible) Simply calling ourselves a Christian is not enough. We need to take to heart that this world (and it's many sinful pleasures) is not forever. Jesus gave a serious warning when He said that on judgement day many will expect to enter heaven, but He will reply “'Depart from me, all you workers of evil!’ And in that place there will be weeping and gnashing of teeth.” (Like 13:27)

The third reason Peter gives for putting away sins we used to live comfortably with is perhaps the best of all. What is that? (vs 18-19)

What amazing love! Using the picture of a helpless, miserable slave being given glorious freedom by someone paying the price for his or her release, Peter reminds his readers the incredible price that was paid for their freedom from sin, death and hell. Jesus Christ, the beloved Son of God through whom creation existed, shed His innocent blood on a cross so that we could be saved from the empty life we inherited from our ancestors.

Verses 20-21 show that this was not some haphazard afterthought on the part of God, but His eternal plan to make the Lord Jesus Christ glorious to all those who are redeemed. How lovely those words are which say to Christians: “Through Christ you have come to trust in God - and you have placed your faith and hope in God because he raised Christ from the dead.” And if that is what has happened in your life, then Peter says to you: “Prepare your minds for action - and exercise self-control. Put all your hope in the gracious salvation that will come to you when Jesus Christ is revealed to the world. Live as God’s obedient children. Don’t slip back into your old ways of living to satisfy your own desires.” Keep that constantly in mind.