Why does God command Christians to be holy when we are already holy in Christ?

“You shall be holy, for I am holy.” - 1 Peter 1:16

"Since we have these promises, beloved, let us cleanse ourselves from every defilement of body and spirit, bringing holiness to completion in the fear of God." 2 Cor 7:1

"...we have been sanctified through the offering of the body of Jesus Christ once for all." Hebrews 10:10

Recently I have come across a number of people online who declare that we cannot become more sanctified ...and we cannot become more like Jesus because we are already perfect in him, so, they reason, that all calls to be holy are wrong.
 
I write this piece because I want to point out how important it is to make distinctions, especially when the Bible does so. In doing so we shall attempt to answer the question(s): Does Christ having made us holy once for all do away with the concept that we are to become more like Him? or does the fact that He has perfected us through the body of Christ once for all (Heb 10:14) contradict the idea that we are to be growing in holiness? How are we to understand all this?  

Perhaps this matter of definitive vs. experiential sanctification can be illustrated and better understood by a story I once heard from a friend.