It is our duty
to be “perfecting holiness in the fear of God,” 2 Cor. 7:1;
to be “growing in grace ” every day, 1 Pet. 2:2, 2 Pet. 3:18;
to be “renewing our inward man day by day,” 2 Cor. 4:16.
Now, this cannot be done without the daily mortifying of sin. Sin sets its strength against every act of holiness, and against every degree we grow to. Let not that man think he makes any progress in holiness who walks not over the bellies of his lusts. He who doth not kill sin in his way takes no steps towards his journey’s end. He who finds not opposition from it, and who sets not himself in every particular to its mortification, is at peace with it, not dying to it.
….sin doth so remain, so act and work in the best of believers, whilst they live in this world, that the constant daily mortification of it is all their days incumbent on them.
– John Owen –
from The Mortification of Sin in Believers, volume 6 of Works, page 14