The notion of a slaveholding church comes as a shock. And it’s not just the Church of England.
The Catholic Church openly advocated colonial conquest and slavery well before Columbus landed in the West Indies and the slave trade got under way. In June 1452, Pope Nicholas V (who in Catholic circles is regarded as a humanist) issued an official letter (Bull) to King Alfonso V of Portugal to permanently enslave any Saracens (Muslims) and pagans, thus facilitating Portuguese slave trade on the West African coast. In January 1455, he issued a longer papal bull to the same King Alfonso, introducing himself as “the Roman Pontiff, successor of the key-bearer of the heavenly kingdom and Vicar of Christ”.

Later comes the crucial bit (the full document - 3537 words - may be read in Ref 1):
“Weighing all the premises with due meditation, …we grant to the aforesaid King Alfonso the full right to invade, search out, capture, vanquish, and subdue all Saracens and pagans whatsoever, and other enemies of Christ wheresoever placed, …and to reduce their persons to perpetual slavery, and to apply and appropriate to himself and his successors the kingdoms,… dominions, possessions, and goods, and to convert them to his and their use and profit…”

Is it the Vicar of Christ speaking or a megalomaniac? Note the utter contempt for non-European peoples. They could be enslaved and dispossessed at will by a decree from Rome.
Taking up this theme, the dissolute Pope Alexander VI (he had 10 children) issued a Bull in 1493 authorising Spain and Portugal the right for “barbarous nations (to) be invaded and brought to the faith”. He took it upon himself to divide the New World between Spain and Portugal.
Pope Leo X (1515) in approval of King Manuel’s conquests presented him with a sword with the words:
“With this, you will wage wars under the happiest auspices… having received through this gift the help of heaven, may you bring back abundant spoils and triumphs.”
Later popes never rescinded these decrees nor apologized. True, there were some mild condemnations of slavery by popes like Paul III (1537) and Gregory XVI (1839) but Catholic Brazil only abolished slavery in 1888, fifty years after its abolition in the British Empire.
 
How do we respond to this perversion of the church's mission? Sri Lankan Catholic theologian, Tissa Balasuriya, has made the attempt. He has the distinction of being excommunicated by the Vatican in 1997 for a book on Mary, mother of Jesus, written from a Third World perspective. The extreme penalty was engineered by none other than German Cardinal Radzinger, then head of the Doctrine of Faith office (formerly the Inquisition), now Pope Benedict XVI. Tissa was reinstated the following year following a world outcry, without repudiating any of his views.

He wrote in the National Catholic Reporter in Jan 1999:
“The principal cause of the present world disequilibrium was the expansion of Western Europe that started in 1492, an expansion based on ‘violence, theft, seizure of land, murder, slavery, genocide and untruth’. The West, by its military, economic and cultural power, continues to siphon off the wealth of the Third World. The Christian churches have to recognize their role in this process. They legitimized it. They benefited from it. They are obliged in justice to return the unjustly acquired benefits.”

On 11 June 2006, Tissa wrote an open letter to the Pope headed “Benedict XVI, rouse yourself!”
It was in reply to the pope’s visit to Auschwitz, the former Nazi death camp, on 28 May 2006. The pope was profoundly moved and prayed: “Rouse yourself! Why do you sleep, O Lord?”
Tissa wrote: “While the Pope is deeply moved by the massacres in Europe, … may we take this occasion to call to mind the similar but wider sufferings of the peoples outside the continent of Europe during the five centuries since 1492?

“Political and even religious leaders of Europe went to the rest of the world as saviours of people ignorant of God and Christ and hence destined to eternal perdition. They claimed to have a mission of civilization… they went with the force of arms and subjugated entire peoples… A theology of slavery legitimized the position of colonialists and capitalists… A very profound re-examination of the theology that dominated Western Christianity during the 1500 years since 325  must be undertaken – the theology that justified actions like the Crusades, the Inquisition, the colonial conquests and setting up of the present unjust world order. It was (is) an exclusivist theology that claimed God to be on the side of the Christians, the chosen people…”
 
References
1. www.romancatholicism.org
2. Tissa Balasuriya, Colonial restitution, National Catholic Reporter, 22 January 1999
3. Tissa Balasuriya, "Benedict XVI, rouse yourself", www.godweb.org/balasuriya.htm (June 2006)