Patrick had converted the people of Antrim, who, thereupon turned away from violent ways, when Coroticus attacked the new Christian community, slaughtered many and took others, - young women, - as slaves, to sell to the Picts of Scotland.
Patrick wrote an epistle to the English Church denouncing this act of piracy and demanding that the young women be returned out of slavery.
... Therefore these words I, with my proper hand,
Have framed and written, for delivery
To these the soldiers' of Coroticus ;
 I say not, to my fellow-citizens,
Nor fellow-citizens of pious Romans,
But rather fellow-citizens of Fiends,
Because of their ill deeds, who, barbarously,
In manner full of hatred, live in death,
 Companions of the Scots and Picts apostate,
Intent to glut their savage souls in blood
Of innocents unnumbered, by myself
In God begotten and in Christ confirmed ... .
... Wherefore, beseech you, all that holy are,(From the translation of Sir Samuel Ferguson)
And all of humble heart, that with such men
Ye hold no flattering converse. You, with them,
Eat not nor drink ; nor of them take their alms,
Until with rigorous penance, and with tears
 Effused, they make atonement, and set free
These new-baptized handmaidens of Christ,
For whom He died and suffered on the cross. ...
Coroticus was a powerful man, a benefactor of the English Church, with many friends in high places. His response to Patrick's attack was as the powerful corrupt always respond, - to launch a vitriolic campaign against the whistleblower's character. He accused Patrick of going native, as becoming as pagan and savage as the Irish themselves, and, indeed, as having attended sun-worshipping ceremonies in his youth. As a consequence charges were formally brought against Patrick and he had to attend before a Synod of bishops in London to answer the charges.
His second, and most famous manuscript, The Confessions of Saint Patrick, was his defence to these charges.
The good bishops of the church and civil authorities in England concerned themselves not with bringing the piratic mass-murderor and slave-trader to justice, but with holding a tribunal into the political correctness of Patrick's ministry.