Most of them had slaves - except, was it Adams, who was Quaker?  You could run one of those large plantations a lot more profitably with slaves than without.

There is an entire section in the OT where how to treat one's slave is explicated in great detail.  Since the instructions and penalties were listed in the bible, a lot of people figured god thought having slaves was just fine.  Someone else will have to look it up for you or you can go to one of the searchable bible web sites.

It all depends on your definition of slavery, I suppose.  An indentured servant whose pay was so low they would never be able to pay off their indebtedness - slave?  A serf who had to give so much of his goods to the local lord of the manor that he and his children starved - slave?  A person who ran the household, living in what was luxury for the times but not paid - slave?  Stuck in Dilbert land - slave?

Jefferson certainly did recognize the hypocrisy and was against the institution of slavery. However, simply "freeing" slaves wasn't that easy. If you freed a slave, what would they do? How would they live? In a letter to Edward Bancroft he wrote

"As far as I can judge from the experiments which have been made, to give liberty to, or rather to abandon persons whose habits have been formed in slavery is like abandoning children"

In a letter to Edward Rutledge he wrote

"I congratulate you, my dear friend, on the law of your State, for suspending the importation of slaves, and for the glory you have justly acquired by endeavoring to prevent it forever. This abomination must have an end. And there is a superior bench reserved in heaven for those who hasten it."

In a letter to Thomas Cooper 

"Do not mistake me. I am not advocating slavery. I am not justifying the wrongs we have committed on a foreign people... On the contrary, there is nothing I would not sacrifice to a practicable plan of abolishing every vestige of this moral and political depravity."

He also recognized the problems and violence that would occur with abolition,

"It will probably be asked, Why not retain and incorporate the blacks into the State [instead of colonizing them]? Deep rooted prejudices entertained by the whites, ten thousand recollections by the blacks of the injuries they have sustained, new provocations, the real distinctions which nature has made, and many other circumstances will divide us into parties and produce convulsions which will probably never end but in the extermination of the one or the other race."

Looking back at history, we can see that the abolition of slavery did lead to decades of racial violence. It took a Civil War and over a hundred years for blacks to gain any semblance of widespread equality. When the country was young could it have survived such a battle? Clearly not. Slaves were an integral part of the (fragile) economy and the country was still licking its wounds from the Revolutionary War. If the states were unable to stay together the British or even one of the other European powers probably would have taken advantage of the weakness. If you read Jefferson's letters that he wrote both before and after writing the Constitution it is quite clear that he was against slavery but did not see a practical way to abolish it without destroying the young country he worked so hard to create. And as horrible as it is, I believe he was right. I don't think our country would be here today had the slavery issue been forced at the founding. 

@ the OP, slavery existed in ancient Greece and also in many hunter-gatherer societies which tended to take slaves (usually women) from groups they conquered in battle. I don't think there is really any correlation between slavery and religion other than popular religions will adjust themselves to fit society. People in the south used the bible to justify slavery just as people now use it against slavery. Christians will make the bible mean whatever they want it to mean and if you read the conflicting words to them you are "taking it out of context" or "you don't understand". Slavery is certainly older than Christianity and may be older than religion or at least organized religion. 

speculate that god's plan is the acquisition of tens of millions of slaves to run his 24-carat household. An eternity of scrubbing the staircases and wiping god's titanic arse might possibly await.