These two quotes from Newton reveal his profound understanding of Christian experience and our sufferings. All genuine believers can bear witness to the truth of these statements.
“When we can fix our thoughts upon him, as laying aside all his honors, and submitting for our sakes to drink of the bitter cup of the wrath of God to the very dregs; and when we further consider, that He who thus suffered in our nature, who knows and sympathizes with all our weakness, is now the Supreme Disposer of all that concerns us, that He numbers the very hairs of our heads, appoints every trial we meet with in number, weight, and measure, and will suffer nothing to befall us but what shall contribute to our good, this view, I say, is a medicine suited to the disease, and powerfully reconciles us to every cross.” (Letters of John Newton, 47-48)
“A lively impression of his love, or of his sufferings for us or of the glories within the vail, accompanied with a due sense of the misery form which we are redeemed; these thoughts will enable us to be not only submissive, but even joyful, in tribulation.” (Letters of John Newton, 210)
Newton is surely correct when he wrote, “How little do the thoughtless and the gay know of that intercourse which passes between believers and the invisible world!” (Letters of John Newton, 201)
 Josiah Bull, ed., Letters by the Rev. John Newton (London: The Religious Tract Society, 1869), 47-48. The Banner of Truth Trust recently published this volume in hardback; you can also download a free pdf version from google books. I find the pdf version to be very versatile for my ipad and computer.