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Yes we were sanctified when born from above, but we must keep ourselves holy in the midst of a fallen world. So sanctification is not a process of making ourselves better or more holy, but of separating from evil and keeping ourselves undefiled. If you believe that a person's sanctification can't become defiled to such an extent that salvation is imperiled, then you are no different than the gnostics who believed the same thing.
Pursue peace with everyone, and holiness without which no one will see the Lord. Hebrews 12:14