Slavery was a way of life all the way until the mid 19th century in the world and beyond, even to these days in isolated places.
No country at the time of the Declaration of Independence was free of slavery of some kind. Christian Europe retained slavery since the Church was recognized as the state institution just shy of 1700 ago. Ergo, no country could have been constituted well. The Church did nothing to stop slavery.
If this was the best system of man,as you say, they certainly knew that it was wrong.This coupled with the way they went about it was despicable by going to another land and praying on the weak.
The Church did nothing to stop slavery.
The Church was politically weak due to the Reformation,they spoke against it,but it went on deaf ears because making money for the wealthy was more important than people in this system.