This is one main reason why the Spirit and the new nature is given unto us,—that we may have a principle within whereby to oppose sin and lust. ” The flesh lusteth against the Spirit.” Well! and what then? Why, “The Spirit also lusteth against the flesh,” Gal. v. 17…. Not to be daily employing the Spirit and new nature for the mortifying of sin, is to neglect that excellent succour which God hath given us against our greatest enemy. If we neglect to make use of what we have received, God may justly hold his hand from giving us more. His graces, as well as his gifts, are bestowed on us to use, exercise, and trade with. Not to be daily mortifying sin, is to sin against the goodness, kindness, wisdom, grace, and love of God, who hath furnished us with a principle of doing it.
– John Owen –
from The Mortification of Sin in Believers; volume 6, of Works, pages 12-13