Answer:
You haven't attached the title of the article.
He was a known abolitionist and advocate of freedom.
Explanation:
"Slave trade" was popular in the 16th century. The British would often trade their goods to Africa for slaves. However, the abolition of the slave trade began in the 1970s through British campaigns. William Wilberforce was one of  abolitionists who became the leader of the parliamentary campaign against it. He became an advocate of freedom due to his belief in his Christian principles.
The author began with a quotation by William Wilberforce to let the readers know that such inspirational quote came from a known abolitionist and advocate of freedom.
So, this explains the answer.