Not literally every one, but the vast majority were slaveowners. The few who weren't took airs to vent about how poor slavery made the whites look, but none of them really had any real
concerns about it.
A lot of people like to call Jefferson an "abolitionist", because he made some such comments in letters, but they only do so by hand-waving the fact that he owned a whole lot of slaves and raped the hell out of at least one and likely a bunch more, over and above the regular brutal beating and mistreatments. And he didn't emancipate any of them, not even his own
children by the one slave we knew he raped repeatedly (this being
how we know), not even on his deathbed.
The Constitution explicitly contained text that described how slavery was to be respected and fleeing slaves were to be returned to their owners, and it was never
directly expunged, just rendered unenforceable after the 13th Amendment so it's generally stricken from modern versions. The Fugitive Slave Clause is the most infamous of these, but not the only time slavery is referenced in the original text;
https://en.wikipedia.org/wiki/Fugitive_Slave_Clause
Even the debates that resulted in the 3/5 compromise had
nothing to do with the indignities and abuses of slavery, and for
both sides, was just about how to secure the most power for their group of States; the slave states wanted slaves to count for Congressional representation (and in no other way be treated as human), because that would mean they got more members in Congress by their population, and the North opposed that because slaves weren't people and allowing that gave the Southern States more Congressional power. That's what the 3/5 Compromise was about; Congressional seating and political power,
not any stance on slavery any supposed rights held by black people.