If “til death do us part” is cryptic, then Christian wedding vows are cryptic. He was saying that as a response to her accepting his marriage proposal. There’s plenty of other stuff to make anyone suspicious of the guy - like returning alone from the trip but not reporting her missing and then lawyering up immediately.
MrsMaw used to watch Unsolved Mysteries. There was a few variations of line they used to use when trying to cast suspicion on the spouse that would make me chuckle. It went something like, “Police began questioning John when they discovered that only eleven years earlier John had taken out a life insurance policy on Lisa.”