What does “For him a debt was the beginning of slavery” mean?
This quote is used to describe the young law student, Marius Pontmercy.
He gave himself the credit of never having owed a farthing to anyone, for to him, a debt was the beginning of slavery.
This means that when you are in debt to someone, that person has a type of control over you. Some debts can lead to slavery, especially if they are large debts and the person cannot pay it back.
The significance of this quote is that it relates to the theme. Many people in the novel have their debts paid by others. Some become slaves because of it, but some are redeemed by it. It reflects the Biblical view that men are all sinners. A holy God requires that someone pay for sins. Men, therefore have a choice in the Christian world view: pay for your sins yourself or accept that Christ paid for your sins by his death on the cross.
A theme of this novel is that sins paid for by Christ (or Christ figures, as in the novel) are done so for free, out of grace, and do not require repayment. Therefore, we are not slaves to debt.
Many authors write about this idea of debt. For example, in Hamlet, Polonius says:
Neither a borrower not a lender be;
For loan oft loses both itself and friend
And borrowing dulls the edge of husbandry.
This is similar to the quote from Les Miserables. The reason for this is that the idea is a Biblical truth: Proverbs 22:7:
The rich rule over the poor, and the borrower is servant to the lender.
See information on the novel here on enotes.