Holy means set apart.. A child of his is Holy because he is his child.. Just like a child of a VIP is considered seperate from most people just because of who their father is.
then their is holiness in how we act. Do we act like our fathers children, He said be holy, as God is holy, We should do this.
Not to stay saved, not to keep salvation, but because we are saved.. This is what empowers us, not the law.. The law can only condemn. and no matter how holy we think we are, even if we are truly a morally upright person.. The law would still condemn us where we stand. Because we are not perfect.