31% of the world's population are Christian, 25% are Muslim, 15% are Hindu, that right there is 71% of the world's population that believe not only in souls, but in immortal souls that continue after death. Plus all the other religions that have souls, or people who aren't particularly religious or spiritual but believe in souls. We as a society have collectively always accepted the existence of souls, and soulless atheist heathens (no value judgment) have always been a minority.
Death is the subject in the phrase. It's from a 16th century Anglican prayer book, The Book of Common Prayer, in which it was "till death us depart," with death being that which would depart (separate) the people making the vow ("us"). However, something that was more common in the 16th century (and is rather more rare in English now though many common phrases still use it), is the subjunctive mood, in which conjugation of verbs has a different form (usually the bare form).