Thomas Aquinas Commentary Jeremiah 31:20

"Is Ephraim my dear son? is he a darling child? for as often as I speak against him, I do earnestly remember him still: therefore my heart yearneth for him; I will surely have mercy upon him, saith Jehovah." — Jeremiah 31:20 (ASV)

Here, the author addresses the penitent person, breaking the topic into three parts:

1. The divine mercy is presented.
2. The instruction for the penitent is given in the command, establish for yourself a watchtower (Jeremiah 31:21).
3. The call for Israel to return is issued: turn, O virgin Israel (Jeremiah 31:21).

Regarding the first point, divine mercy, the author makes two observations.

First, he gives the reason for mercy from the people's perspective. God asks the rhetorical questions, is Ephraim a son who brings me honor? and is he a delicate boy? The first question alludes to Ephraim's birthright, and the second to the tenderness of his age, because Joseph was, in a sense, the youngest of Jacob's sons,Only until the birth of Benjamin; cf. Genesis 35:24. and Ephraim was the younger of Joseph's sons. These two factors—honor and youth—move fathers to show mercy, as it is written: Israel was a boy, and I loved him, and from Egypt have I called my son (Hosea 11:1).

The author also gives the reason for mercy from God's perspective: His own faithfulness. He quotes, for from the point when I spoke, I shall remember the words. This is as if to say, "It is not fitting that God's words should be made void." This aligns with the principle: Has he said, and will not do it? Has he spoken and will he not fulfill it? (Numbers 23:19).

Second, the author shows how this mercy is expressed. The phrase therefore, are my inward parts moved is an example of anthropopathism,A literary device whereby human affections are ascribed to a non-human being, in this case, God. which signifies internal compassion. The promise I shall surely have mercy signifies the external act of bringing aid. This is supported by another passage: My heart is turned within me; likewise my repentance is stirred up (Hosea 11:8).