What does John Wesley mean by this phrase?

The Latin phrase translates as “Athanasius against the world.” Athanasius was a Christian theologian, a Church Father, and the chief defender of Trinitarianism against Arianism. He battled against the Arian heresy prevalent in the fourth century and his persistence resulted in the defeat of his detractors. Here is a brief overview:

In 325, at the age of 27, Athanasius began his leading role against the Arians as a deacon and assistant to Bishop Alexander of Alexandria during the First Council of Nicaea. Roman emperor Constantine the Great had convened the council in May–August 325 to address the Arian position that the Son of God, Jesus of Nazareth, is of a distinct substance from the Father. Three years after that council, Athanasius succeeded his mentor as archbishop of Alexandria. In addition to the conflict with the Arians (including powerful and influential Arian churchmen led by Eusebius of Nicomedia), he struggled against the Emperors Constantine, Constantius II, Julian the Apostate and Valens. He was known as Athanasius Contra Mundum (Latin for Athanasius Against the World). https://en.wikipedia.org/wiki/Athanasius_of_Alexandria

Background Information on John Wesley and the Methodists: Under the direction of John Wesley, Methodists became leaders in many areas of social justice, including prison reform and the abolition of the Slave Trade. John Wesley’s letter to William Wilberforce was written six days before his death. From 1739 onwards, Wesley and the Methodists were persecuted by clergymen and magistrates. They were attacked in sermons and in print and at times attacked by mobs because of their stand against slavery.

In 1788, when the abolition campaign was at its height, he preached a sermon in Bristol, one of the foremost slave trading ports. In those days, an anti-slavery sermon could not be preached without considerable personal risk to the preacher and a disturbance broke out.

He maintained an interest in the abolition movement until he died. Wesley also famously said:

"Give liberty to whom liberty is due, that is, to every child of man, to every partaker of human nature. Let none serve you but by his own act and deed, by his own voluntary action. Away with all whips, all chains, all compulsion. Be gentle toward all men; and see that you invariably do with every one as you would he should do unto you." http://abolition.e2bn.org/people_32.html

Background Information on William Wilberforce: William Wilberforce made many enemies during his campaign to have slavery abolished. In 1807 he published a 350-page book, ‘Letter on the Abolition of the Slave Trade’, part of which said this:

That the Almighty Creator of the universe governs the world which he has made; that the sufferings of nations are to be regarded as the punishment of national crimes; and their decline and fall, as the execution of this sentence; are truths which I trust are still generally believed among us … If these truths be admitted, and if it be also true, that fraud, oppression and cruelty, are crimes of the blackest dye, and that guilt is aggravated in proportion as the criminal acts in defiance of clearer light, and of stronger motives to virtue… have we not abundant cause for serious apprehension?… If… the Slave Trade be a national crime… to which we cling in defiance of the clearest light, not only in opposition to our own acknowledgements of its guilt, but even of our own declared resolutions to abandon it, is not this, then a time at which all who are not perfectly sure that the Providence of God is but a fable, should be strenuous in their endeavours to lighten the vessel of the state, of such a load of guilt and infamy? (Letter on the Abolition of the Slave Trade, 1807, pp. 4–6) Source: https://www.open.edu/openlearn/history-the-arts/history/history-art/wilberforce/content-section-5.1

Here is an extract from a link that includes John Wesley’s letter to William Wilberforce:

WRITTEN FEBRUARY 24, 1791 at age 88 from Balam, England, six days before his death, this last letter of Wesley’s was addressed to William Wilberforce. Wesley had spoken out forcibly against slavery, repeatedly referring to the slave trade as the “execrable sum of all villainies”. In 1774 he wrote the influential Thoughts Upon Slavery. Wilberforce, a Member of Parliament, was active at the time in an unsuccessful attempt to pass abolition. Debate continued for several years and in 1807 the abolition of slavery was effected throughout the British Empire.

The text of the letter is given below and can be used to follow the aged, faltering hand of the still hearty Wesley. The “tract” to which Wesley refers was written by a former slave, Gustavus Vassa, who was born in 1745 in Africa, kidnapped and sold for a slave in Barbados. In 1757 he was sent to England and, according to church records, was soon converted to Christianity. https://christianhistoryinstitute.org/magazine/article/wesley-to-wilberforce/

Context of “Athanasius contra Mundum” with regard to the anti-slavery crusade: John Wesley knew, from first-hand experience, that the battle against slavery had made him many enemies. He also knew that William Wilberforce was fighting against powerful politicians and merchants, many of whom had a vested financial interest in maintaining the slave trade. That is why Wesley made reference to Athanasius, that fourth century theologian who waged war against the religious establishment in order to root out heresy. He was acknowledging that Wilberforce was up against the entire establishment, both political and commercial, and that, like Athanasius, he would be fighting against the world in his endeavour to bring slavery to an end.

His prayer for Wilberforce was that God, who had guided him from his youth, would continue to strengthen him in this mighty battle. Wesley’s prayer for Wilberforce was answered.