Use of "upon" or "on" in phrase

Question

In a spiritual phrase the segment says

...have mercy on me, a sinner.

could you use "upon" rather than "on"?

I feel that using "upon" personalizes the phrase better.

Or is my grammar failing?

Answer 1

"On" and "upon" are interchangeable most of the time. Not always.

He once wrote a book on Shakespeare's sonnets.

Can't use "upon" here.

Upon sounds a notch more formal and can be a lot more emphatic, depending on the context.

There's some wild talk about "upon" only being used where actual physical space is involved, but it doesn't make much sense: "Upon my honor" is a perfectly legitimate phrase.

To confuse matters further, you can (and are encouraged to) use onto where both space and action are involved:

He climbed onto his horse.

Climbing upon one's horse is permissible, but climbing on does sound a bit comical (evoking, as it does, the image of a rider doing some climbing while already sitting on his horse).

Answer 2

"'The Jesus Prayer' (Greek: Η Προσευχή του Ιησού, i prosefchí tou iisoú; Slotho d-Yeshu') or 'The Prayer' (Greek: Η Ευχή, i efchí̱ – literally "The Wish") is a short, formulaic prayer esteemed and advocated especially within Eastern [Orthodox] Churches:

'Lord Jesus Christ, Son of God, have mercy on me, the sinner.'”

From: https://en.wikipedia.org/wiki/Jesus_Prayer

As such, I would not advise messing with it to make it more "personalized." But that is just my opinion, and you are free to change it as you please. Good luck!