What is the significance of using ἀγάπησιν rather than agapao in the Septuagint?

Question

Some say the use of ἀγαπάω (agapao) in the New Testament must be understood as the exclusive verb form of ἀγάπη (agape). The Septuagint does not appear to reflect this understanding.

In the LXX, the use of agape is limited: only 14 verses, 10 in the Song of Solomon. The LXX translators also used ἀγάπησιν, another form of agape, in 8 places. For example in David's lament over the deaths of Saul and Jonathan:

I am distressed for you, my brother Jonathan; very pleasant have you been to me; your love to me was extraordinary, surpassing the love of women. (2 Samuel 1:26 ESV)

ἀλγῶ ἐπὶ σοί ἄδελφέ μου Ιωναθαν ὡραιώθης μοι σφόδρα ἐθαυμαστώθη ἡ ἀγάπησίς σου ἐμοὶ ὑπὲρ ἀγάπησιν γυναικῶν

The LXX translator used ἀγάπησίς not agapao. In this case, apparently the translator understood a meaning of the noun agape which the verb agapao did not correctly convey and used of a different form of agape, ἀγάπησίς (a word not used in the New Testament).

In Jeremiah, both ἀγάπησιν and ἠγάπησά are used:

the Lord appeared to him from far away. I have loved you with an everlasting love; therefore I have continued my faithfulness to you. (31:3 ESV)

κύριος πόρρωθεν ὤφθη αὐτῷ ἀγάπησιν αἰωνίαν ἠγάπησά σε διὰ τοῦτο εἵλκυσά σε εἰς οἰκτίρημα

Again, apparently the translator understood a meaning of agape which agapao did not convey and used a different word.

The use in the Septuagint demonstrates the verb agapao is not the exclusive means to express the meaning of the noun agape. This raises questions:

1. What is the significance of using ἀγάπησιν rather than agapao in the LXX? Does this demonstrate a wider range of meaning of agape which required 2 different forms to express?
2. Given the use in the LXX should agapao be considered as the exclusive means to express agape in the New Testament? Or did writers simply choose to express the "other than agapao" meaning of agape differently than the LXX? (For example, rather than use a single word like ἀγάπησιν, they gave examples and detailed descriptions.)

Answer 1

I don't think anything in Septuagint usage contradicts the idea, as challenged by the OP, that:

ἀγαπάω (agapao) must be understood as the verb form of ἀγάπη (agape)

(Note that the verb is actually the older form; ἀγάπη and ἀγάπησις are deverbal nouns.)

To address each example in turn:

2 Sam 1:26

נִפְלְאַתָה אַהֲבָתְךָ לִי מֵאַהֲבַת נָשִֽים
ἐθαυμαστώθη ἡ ἀγάπησίς σου ἐμοὶ ὑπὲρ ἀγάπησιν γυναικῶν
Your love to me was more wonderful than love of women.²

Here the same Greek noun is used twice, each translating the same Hebrew noun. I do not see how this can be used as evidence that

[OP]: the translator understood a difference between the verb agapao and the noun agape which required the use of a different form of agape

The verb is not involved at all here in either Hebrew or Greek. The fact that the noun ἀγάπησις rather than ἀγάπη is used could be explored, but this is not informative with respect to the verb.¹

Jer 31:3

וְאַהֲבַת עוֹלָם אֲהַבְתִּ֔יךְ
Ἀγάπησιν αἰωνίαν ἠγάπησά σε
With an everlasting love I have loved you.²

In this example, noun and verb forms are indeed used together. The word choices again arise directly from the source language. Hebrew is using a cognate noun phrase as an adjunct to the verb, a very common Semitic way to express adverbial ideas. Geek has simply reproduced this. If anything, this demonstrates well the cognate relationship between noun and verb which the OP doubts.

[OP]: Again, the translator understood a meaning of the verb which was not exactly the same as the noun...

As noted in the comments, verbs and nouns can't mean "exactly the same thing", but this is a standard usage of cognates.

[OP, cont.]... and used two forms of the noun.

I'm not sure what you're seeing here. ἀγάπησις is used only once. ἀγάπη is not used.

---

_{1. Per NIDTTE*, the "cognate ἀγάπησις [has] no apparent difference in meaning". This entry additionally points out that both nominal forms are late constructions which appear rarely outside LXX, NT, and dependent literature.}

_{2. English is NASB but would be a fine translation of the Greek as well in both cases.}

_{* ἀγαπάω in New International Dictionary of New Testament Theology and Exegesis (NIDTTE). Ed. Moises Silva (Zondervan, 2014), 1:102.}