There is a faith-sustained holiness that Paul wants his converts to have on the day of Christ — the day of his return, when the dead in Christ will be raised (1 Corinthians 15:23). This holiness (which he also calls “blamelessness” and “guiltlessness” and “being above reproach” and “purity”) is certain through God’s faithfulness, contingent on persevering faith, and dependent on human agency.
Paul is certain that God will work this persevering faith and holiness in his converts for the day of Christ. This is part of God’s faithfulness.
May the God of peace himself sanctify you completely, and may your whole spirit and soul and body be kept blameless at the coming of our Lord …