Here are a few milestones in the Church's slow rousing to oppose slavery:
1315 - King Louis X abolished slavery in France (but it would later be reestablished)
1435 - Pope Eugene IV, “Sicut Dudum”, opposed slavery in the Canary Islands
1504 - Queen Isabella of Castile, then of Spain, in her last will forbids the enslavement of the indigenous peoples of the New World
1537 - Pope Paul III, “Sublimus Deus”
1591 - Pope Gregory XIV, “Bulla Cum Sicuti”, ordered payment of reparations to enslaved Philippine natives
1639 - Pope Urban VIII, “Commissum Nobis”
1686 - Pope Innocent XI condemned slavery due to the influence of Lourenço da Silva Mendouça, a member of the royal family of the Kingdom of Ndongo in what is now Angola
1741 - Pope Benedict XIV, “Immensa Pastorum”
1807 - British Parliament, “An Act for the Abolition of the Slave Trade”
1808 - US Congress, “Act Prohibiting the Importation of Slaves”
1815 - Pope Pius VII at the Congress of Vienna
1839 - Pope Gregory, “In Supremo”
1863 - Lincoln’s Emancipation Proclamation
Prior to 1440 AD, select theologians and lower church officials opposed slavery, plus one French king, whose successors allowed slavery to return. From 1440-1660, you see the beginning of top-down pressure from monarchs and popes to limit or outlaw slavery. These efforts had modest results. Then from 1680-1800, calls for abolition began to capture the public imagination. Finally, from 1800 onward slavery was forcefully opposed and rooted out worldwide.