"Believers should first be assured that sanctification, like justification, is grounded in union with Christ. The power of sin to rule their lives has been destroyed in the Cross of Christ; we have died with Christ, and have been raised up together with Him in newness of life. Therefore we are not to set the estimates of our power to conquer sin according to past experience of our will power, but are to fix our attention on Christ and the power of His risen life in which we participate; for we have died and our life is hidden with Christ in God. This power is accessible in our experience through faith, not through simple striving of the will."
Therefore, we grow in holiness by tapping into the grace and power of Christ made available to us through our union with Him, which we have by faith.