Why is the verb "do" used in the phrase, Till death do us part, and not "does"?

I also looked in The Free Dictionary, but it didn't explain why

till death do us part
A common phrase used in wedding ceremonies indicating that the marriage bond is be intended to last until death. Taken from the marriage liturgy in the Anglican Communion's Book of Common Prayer, the phrase refers to the fact that marriage is seen in the Christian faith as insoluble except by the death of one of the partners. (Also worded as "till death us do part," which is how it is written in the Book of Common Prayer.)