Being and still becoming holy

The only way something which is not holy becomes holy is through God making it holy.  In Leviticus 20:26, God says to His people,

You shall be holy to me, for I the Lord am holy and have separated you from the peoples, that you should be mine.  Leviticus 20:26

It was God who separated His people from everyone else.  We do not become holy through our own initiative.  Holiness is dependent on God's action on our behalf.  This is the heart of the gospel.  Hebrews 10:10 says, "And by that will [that is, the will of God], we have been made holy through the sacrifice of the body of Jesus Christ once for all."

Notice verse 10 says "we have been made holy".  Now read verse 14:

14 For by one sacrifice he has made perfect forever those who are being made holy.

So in verse 10 "we have been made holy" and in verse 14 we "are being made holy".  This is the beauty of the gospel.  Through our faith in Christ we have both been made holy and yet still are being made holy.  How does this work?

The two key words you should know are justification and sanctification.

At the moment we first believed the gospel that Christ, the Son of God, came, died for our sins and was resurrected we became justified.  Christ's perfect obedience became our perfect obedience.  We were "made perfect forever" (vs 14).  The sin which made us unholy was forgiven because the penalty for our sin was fulfilled by Christ.  At that moment God became for us (He was previously against us), and God set us apart for His purposes (made us holy).

Our justification began the process of our sanctification in which we are now gradually being conformed to be like Christ.  Romans explains sanctification this way:

1 I appeal to you therefore, brothers, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship. 2 Do not be conformed to this world, but be transformed by the renewal of your mind, that by testing you may discern what is the will of God, what is good and acceptable and perfect.  Romans 12:1-2

So once God set us apart (made us holy) we began the process of becoming more like Him (are being made holy).