One of the main sources of confusion in common phrases is the prevalence of archaic and obsolete words, some of which only linger on because of their presence in these phrases. That’s the case with one of today’s words, a word that, apart from a few cliche terms, you’ll most likely never encounter outside of a church. It’s therefore completely reasonable that people would substitute the more modern word.
Lo: an expression meaning “Look! See!”
Low: Close to the ground.
Lo is an old word, first showing up around 900. It’s from an Old English word, la, an exclamation of surprise, grief or joy. Another Old English word, loke, meaning look, has been shortened to lo, the two words being conflated into one.
Low is of a bit more recent vintage, originating around 1150. It makes its way to Middle English from the Old High German laege, which means to lie (lying down, not telling a fib.)
If you quote Jesus saying “low, I tell you…” you’re suggesting that he’s doing his Barry White impression.