Whilst staying at the assylum he opened a Bible and discovered Romans 3:25, "whom God put forward as a propitiation by his blood, to be received by faith. This was to show God’s righteousness, because in his divine forbearance he had passed over former sins." Here is how Cowper describes the moment:
Immediately I received the strength to believe it, and the full beams of the Sun of Righteousness shone upon me. I saw the sufficiency of the atonement He had made, my pardon sealed in His blood, and all the fullness and completeness of His justification. In a moment I believed, and received the gospel ... Whatever my friend Madan had said to me, long before, revived in all its clearness, with demonstration of the spirit and power. Unless the Almighty arm had been under me, I think I should have died with gratitude and joy. My eyes filled with tears, and my voice choked with transport; I could only look up to heaven in silent fear, overwhelmed with love and wonder.
(Gilbert Thomas, William Cowper and the Eighteenth Century, p. 132.)