On the evening of March 15, 1796, the streets adjacent to the Opera House of London thronged with carriages carrying the wealthy and powerful. Excited conversation buzzed beneath wrought-iron streetlamps. It was the premiere of the opera season. Vignoni, the noted Italian singer, was the lead in a new comic opera, I Dui Gobi—his first London performance since France had declared war on Britain in 1793. There was every reason to think that the night would be a triumph—and it was. True Briton reported that the premiere had been attended by "a large and splendid audience."

But March 15 also marked a triumph of a more insidious kind—a triumph that nearly ended the parliamentary career of the man who has been called "the greatest reformer in history": William Wilberforce.

On the very night of the I Dui Gobi premiere, a crucial vote took place in the House of Commons on Wilberforce's bill to abolish the British slave trade. The debate would decide the fate of this bill—whether countless sons and daughters of Africa would continue to be enslaved, or whether the British traffic in human flesh would end. For several weeks leading up to the March 15 debate, there were promising signs for a successful conclusion to what had been Wilberforce's consuming passion for nine years.

His opponents, however, hit upon a subtle tactic to shipwreck the bill. Capitalizing on the popularity of the opera premiere, they made sure free tickets were provided for Wilberforce's more lukewarm supporters. The sabotage attempt worked. In a terse diary entry, Wilberforce described his devastating defeat: "My Slave Bill was thrown out by 74 to 70. … Ten or twelve of those who had supported me [were] absent in the country, or [away] on pleasure. Enough [were] at the Opera to have carried it."

Wilberforce's hopes were crushed. He contracted a severe fever, followed by excruciating intestinal troubles. Gravely ill, exhausted, and emotionally spent, he suffered what appears to have been his second nervous breakdown. His catastrophic defeat and debilitating illness prompted thoughts of retirement from public life. Seeking counsel, on July 21 he wrote a letter to his old friend and spiritual mentor John Newton—the former-slave-trader-turned-clergyman known today as the author of the hymn "Amazing Grace."

Newton's reply was eloquent and imbued with great wisdom. If God had not yet allowed Wilberforce to be dismissed from Parliament, he should take that as a "token for good." "Some of [God's] people may be emphatically said not to live to themselves. May it not be said of you? … You meet with many things which weary and disgust you … but then they are inseparably connected with your path of duty; and though you cannot do all the good you wish for, some good is done."

Just as Esther had been put in the palace of King Xerxes "for such a time as this," Newton wrote, "one may not be able to calculate all of the advantages that may result from your service in public life. The example, and even the presence of a consistent character, may have a powerful, though unobserved, effect upon others. You are in a place where many know Him not, and can show them the genuine fruits of the religion you are known to profess."

Newton wisely reminded Wilberforce that he had already accomplished a great deal: "Though you have not, as yet, fully succeeded in your persevering endeavours to abolish the slave trade, since you took it in hand the condition of the slaves in our islands has undoubtedly been already improved." These improvements were "proofs that you have not laboured in vain."

Newton next pointed to another biblical character who had served long, faithfully, and well under trying circumstances. Like Daniel, Wilberforce knew the loneliness of being a trailblazer. "It is true," Newton conceded, "that you live in the midst of difficulties and snares, and you need a double guard of watchfulness and prayer. But since you know both your need of help, and where to look for it, I may say to you as Darius to Daniel, 'Thy God whom thou servest continually is able to preserve and deliver you.' Daniel, likewise, was a public man, and in critical circumstances; but he trusted in the Lord; was faithful in his department, and therefore though he had enemies, they could not prevail against him.

"Indeed," Newton continued, "the great point for our comfort in life is to have a well-grounded persuasion that we are where, all things considered, we ought to be. Then it is no great matter whether we are in public or in private life, in a city or a village, in a palace or a cottage. The promise, 'My grace is sufficient for thee,' is necessary to support us in the smoothest scenes, and is equally able to support us in the most difficult.

"Happy the man who has a deep impression of our Lord's words, 'Without Me you can do nothing'—who feels with the Apostle … likewise a heartfelt dependence upon the Saviour.

"He is always near. He knows our wants, our dangers, our feelings, and our fears. By looking to him we are made strong out of weakness. With his wisdom for our guide, his power for our protection and his fullness for our supply, we shall be able to 'withstand in the evil day, and having done all to stand.'

"May the Lord bless you. May he be your sun and shield, and fill you with all joy and peace in believing."

Newton's words gave Wilberforce the solace and strength he so desperately needed. He would remain in politics. He would remain in the battle to end the slave trade.

Neither man knew that 11 long years would pass before the goal was finally reached. The slave trade was legally abolished in the spring of 1807. Newton died the same year, but not before hearing the news from his friend Wilberforce, the defeated crusader who had found strength beyond himself to keep fighting.

Kevin Belmonte is the author of the award-winning biography William Wilberforce: A Hero for Humanity (now available from Zondervan). For the past five years, he has served as the lead historical consultant for the major motion picture Amazing Grace: The William Wilberforce Story.