Mortification of Sin
I’ll let John Owen speak for himself:
An unmortified lust will drink up the spirit and all the vigour of the soul, and weaken it for all duties […] It untunes and unframes the heart itself, by entangling its affections. It diverts the heart from that spiritual frame that is required for vigorous communion with God. It lays hold on the affections, rendering its object beloved and desirable, and so expelling the love of the Father (1 John 2:15; 3:17). So that the soul cannot say uprightly and truly to God, ‘Thou art my portion’, having something else that it loves. Fear, desire, hope, which are the choice affections of the soul, that should be full of God, will be one way or other entangled with it. (The Mortification of Sin; Christian Focus edition, pp. 51-52).