Not true. While the Church gradually and over time succeeded in largely banning the enslavement of Christians by other Christians, enslavement of non-Christians remained permissible throughout the middle ages and beyond. The Papal States used to use Muslim slaves in their galleys. Even the ban on enslaving Christians was not consistently applied throughout the period.
I'd add that even if slavery of Christians by Christians was formally condemned, many many people until XIX centuries were de facto enslaved because of their debts so working in order to repay them (it'd take even decades or the entire lifespan) was normal and accepted. The debtor was a prisoner by all means, he lost all his right belonging to his creditor. Obviously once a debtor started to work it was very hard that he will set free one day because the rules were made to trap him in a spiral of continuos debts towards his master.
