A pledge was a guarantee.
Security for a debt. A hostage.
It shows you trust the other person not to kill them.
This is one evolution of that.
If someone invites you to drink and you drink what they give you, that is the sign of trust that they will not poison you.
You are drinking their wine, pledging your faith. If I drink first it means I trust you. I pledge you, then you pledge me. Reciprocal trust.
Quote:(transitive) To give assurance of friendship by the act of drinking; to drink to one's health.
1773, Oliver Goldsmith, She Stoops to Conquer:
HARDCASTLE [Taking the cup.] I hope you'll find it to your mind. I have prepared it with my own hands, and I believe you'll own the ingredients are tolerable. Will you be so good as to pledge me, sir? Here, Mr. Marlow, here is to our better acquaintance. [Drinks.]
1852, Matthew Arnold, Tristram and Iseult:
Reach me my golden cup that stands by thee,
And pledge me in it first for courtesy.
Then further evolution. To propose a drink to honour someone.
A man who refuses to drink to Richard is dishonourable.
I drink this goblet to honour the great man.
So the trust is the one
accepting the drink. But in this case, the meaning has moved on to the idea of toasting someone. Not fear of poison.
Drinking is not a common modern meaning.
It is still ceremonial language "I pledge the health of" for proposing a toast.
[Toast was originally the spiced bread put in the drink. But it later became the act of raising your glass and taking a drink. But that move is recent, eighteenth century]
Pledge also moved on to mean to swear, to promise to do something. Instead of goods or hostages as security you were giving
yourself, your word, your honour. In that meaning, it includes "taking the pledge" - the promise to not drink alcohol. And things like a pledge of allegiance.
But it still includes the original hostage or security for a debt, because you 'pledge' items you put in hock to a pawnbroker. Legally, in English law, anything given as security for a debt is a pledge.