In 1791 John Wesley, the founder of the Methodist movement, wrote to William Wilberforce to encourage him in his fight against slave trade.
In his biography of Wilberforce, Eric Metaxas makes this observation on Wesley's advice:
"The ... point Wesley was making was that it was not merely a political or a cultural battle. It was a spiritual battle. When Wesley wrote, 'you will be worn out by the opposition of men and devils,' he was not using a colorful metaphor. He meant it literally.
To fight something as wicked as the slave trade was to go against an invisible demonic host. God has the power to fight them, but we do not. That spiritual reality lay behind the political reality, and Wesley wanted to ensure that Wilberforce understood that if he was to be successful in what lay ahead. Great men like Wilberforce and Wesley had the humility and wisdom to know that whatever strengths they had - and they had many - they could not win without a total reliance on God. At its core, every battle worth fighting is a spiritual battle. Those men were able to succeed only because they humbled themselves and entrusted the battle to God.
But how does one do that?
This brings us to the second way that Wilberforce did what he did. The one-word answer is prayer. Wilberforce prayed and read the Scriptures every day, and he prayed with many others over these issues and concerns."
Eric Metaxas, Seven Men and the Secret of their Greatness, p48.