IMO, it simply means that to "swear a vow" is to put question on everything else you say. Like if you say "I promise I'm telling you the truth", does that mean that at some point before, you weren't? I've studied a lot of religions and beliefs, and one conclusion I can come to from all of them, no "enlightenment" or "holiness" will come from secrets or lies. If we are truly like Christ, then everything we say should be truth. That doesn't mean you have to completely fact check every thing you ever say, it simply means that if you say you are gonna do something, then you better do it, because to do otherwise is a lie.
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