God's grace is unconditional, but there are strings attached.
Read / Listen to the chapter:
Read the chapter in an outlined format
1 Thessalonians 4 (ESV)
1 Finally, then, brothers,
we ask and urge you
in the Lord Jesus,
that as you received from us how you ought to walk and to please God,
just as you are doing,
that you do so more and more.
2 For you know what instructions we gave you through the Lord Jesus.
3 For this is the will of God, your sanctification:
that you abstain from sexual immorality;
4 that each one of you know how to control his own body
in holiness and honor,
5 not in the passion of lust like the Gentiles who do not know God;
6 that no one transgress and wrong his brother in this matter,
because the Lord is an avenger in all these things,
as we told you beforehand and solemnly warned you.
7 For God has not called us
but in holiness.