The Wealth of Nations by Part 3 Chapter 2 Page 19

partly upon account of this advantage, and partly upon account of the encroachments which the sovereign, always jealous of the great lords, gradually encouraged their villains to make upon their authority, and which seem at last to have been such as rendered this species of servitude altogether inconvenient, that tenure in villanage gradually wore out through the greater part of Europe. The time and manner, however, in which so important a revolution was brought about is one of the most obscure points in modern history. The Church of Rome claims great merit in it; and it is certain that so early as the twelfth century, Alexander III published a bull for the general emancipation of slaves.

It seems, however, to have been rather a pious exhortation than a law to which exact obedience was required from the faithful.