The Christian calling, like many others, is easy and clear in theory, but not without much care and difficulty to be reduced to practice. Things appear quite otherwise, when felt experimentally, to what they do, when only read in a book. . . . So, to renounce self, to live upon Jesus, to walk with God, to overcome the world, to hope against hope, to trust the Lord when we cannot trace him, and to know that our duty and privilege consist in these things, may be readily acknowledged or quickly learned; but, upon repeated trial, we find that saying and doing are two things. We think at setting out that we sit down and count the cost; but alas! our views are so superficial at first, that we have occasion to correct our estimate daily. For every day shows us some new thing in the heart, or some new turn in the management of the war against us which we are not aware of; and upon these accounts, discouragements may arise so high as to bring us (I speak for myself) to the very point of throwing down our arms, and making either a tame surrender or a shameful flight. Thus it would be with us at last if the Lord of hosts were not on our side. . . But if He is the Captain of our salvation, if his eye is upon us, his arm stretched out around us, and his ear open to our cry, and if He has engaged to teach our hands to war and our fingers to fight, and to cover our heads in the day of battle, then we need not fear, though a host rise up against us; but lifting up our banner in his name, let us go forth conquering and to conquer (Rom 16:20).
–John Newton, “To William Cowper,” Letter I. 30 July 1767. Letters of John Newton.