This phrase is the central command of 1 Peter 1:14–16, upon which the rest of the material of these three verses turn. They are instructed to “be holy”
because they are now obedient children of God
no longer being conformed to their passions as they used to be
in all they do
in obedience to the Scriptures
15 but as he who called you is holy, you also be holy in all your conduct,