In the Hoshana poem L'ma'an Amitach, various characteristics of God are enlisted as a reason for God to save us. The version appearing in my prayerbook, and recited in congregations I have been present at, lists the following characteristics:
- Truth (אמתך)
- Covenant (בריתך)
- Greatness and splendor (גדלך ותפארתך)
- Law (דתך)
- Grandeur (הודך)
- Assembly (ועודך)
- Remembrance (זכרך)
- Kindness (חסדך)
- Goodness (טובך)
- Oneness (יחודך)
- Honor (כבודך)
- Learning (למודך)
- Kingship (מלכותך)
- Eternity (נצחך)
- Secret (סודך)
- Might (עזך)
- Glory (פארך)
- Righteousness (צדקתך)
- Holiness (קדשתך)
- Abundant mercy (רחמיך הרבים)
- Divine presence (שכינתך)
- Praise (תהלתך)
There seems to be a very obvious "odd man out" here. The third-to-last one is the only one in which the characteristic is quantified. We ask God to save us, not on account of his mercy but on account of his abundant mercy. We don't similarly quantify God's holiness, honor, righteousness, kindness, etc.
Why do we specifically quantify God's mercy but not any of the other characteristics? (Obviously, not all of the characteristics can be modified in this fashion, but there are quite a few of them that can be.)
I am aware that the term "abundant mercy" appears in a verse in Daniel 9:18, but I don't know if that is relevant to this.
I would also note, that in other prayer contexts "mercy" is not uniquely quantified. For instance, in the Avinu Malkeinu prayer, my edition has "Act for the sake of your abundant mercy" immediately followed by "act for the sake of your great, mighty, and awesome name". Or, for example, in this Yemenite prayerbook, a similar list of God's characteristics does not quantify "mercy" any differently from the others:
