David Peterson in his book, Possessed by God: A New Testament Theology of Sanctification and Holiness explains how Christ followers are positionally sanctified in Christ:
Our essential identity as Christians is formed by Christ and the gospel, not by our own personalities, backgrounds or achievements. Through the death and resurrection of his Son, God has cleansed us from the guilt of sin and liberated us from its consequences and its control. He has set us in a right and faithful relation to himself, together with all who call upon the name of our Lord Jesus Christ. Drawing us into an exclusive relationship with himself in this way, he has made us his holy people, destined to serve him and please him for ever. Sanctification is about being possessed by God and expressing that distinctive and exclusive relationship by the way we live.
Although God calls upon us to express the fact that we have been sanctified by the way we live, our standing with him does not depend on the degree to which we live up to his expectations. It depends on his grace alone. Those who are bowed down by the pressure of temptation and an awareness of failure need to be reminded of the definitive, sanctifying work of God in Christ, by which he has established us as his holy people. On this basis, they should be urged to press on in hope and grasp again by faith the benefits of Christ’s sacrifice. Approaching the exalted Lord with boldness, we may always receive mercy and find ‘grace to help in time of need’ (Heb. 4:16), (pg. 47–48).