"So brace up your minds, and, as those who know what they are doing, rest the full weight of your hopes on the grace that will be yours when Jesus Christ reveals himself. Live as obedient children before God. Don’t let your character be molded by the desires of your ignorant days, but be holy in every department of your lives, for the one who has called you is himself holy. The scripture says: ‘Be holy, for I am holy.'" 1 Peter 1:13-16
Which, with those last words, we can begin to register as another of those scriptural impossibilities unless we consider, here, the context of what's just been said. In the third sentence, what are we told not to do? "Don't let your character be molded by the desires of your ignorant days..." Which, if it sounds familiar, almost perfectly echoes Romans 12:2 - "Don’t let the world around you squeeze you into its own mold..."
So how are we to avoid these "wrong moldings" of our lives and character? Well, let's read the remainder of Romans 12:2 - "...but let God re-mold your minds from within, so that you may prove in practice that the plan of God for you is good, meets all his demands and moves towards the goal of true maturity.” Which accords perfectly with the ending of our section in 1 Peter: "...but be holy in every department of your lives, for the one who has called you is himself holy. The scripture says: ‘Be holy, for I am holy.'"
So, since the One who lives within us is righteous and holy, the writer of a plan for our lives that is good, it is daily submission to Him that defeats the cookie-cutter moldings that this world is after for each of us. Or to put it another way: Our lives will be molded, one way or the other: but the choice is, really, truly, in our hands and hearts which way that goes...