NIV 1 John 4:8
Whoever does not love does not know God, because God is love.
Knowing that Greek has multiple words for love which sense of love is employed here in the last half of the verse?
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The Nestle-Aland critical text of 1 John 4.8:
ὅτι ὁ θεὸς ἀγάπη ἐστίν
The Greek word used is agapē. Middle-Liddel defines this as:
I. love: esp. brotherly love, charity; the love of God for man and of man for God, NTest.
II. in pl. a love-feast, id=NTest.
LSJ expands this:
A. love, LXX Je.2.2, Ca.2.7, al.; “ἀ. καὶ μῖσος” Ec.9.1; dub. l. in PBerol.9859 (ii B. C.<*>, Phld.Lib.p.52 O; of the love of husband and wife, Sch.Ptol.Tetr.52.
- esp. love of God for man and of man for God, LXX Wi.3.9, Aristeas 229; “φόβος καὶ ἀ.” Ph.1.283, cf. Ep.Rom.5.8, 2 Ep.Cor.5.14, Ev.Luc.11.42, al.:—also brotherly love, charity, 1 Ep.Cor.13.1, al.
II. in pl., love-feast, 2 Ep.Pet. 2.13, Ep.Jud.12.
III. alms, charity, PGen.14 (iv/v A. D.).
IV. ἀγάπη θεῶν, title of Isis, POxy.1380.109 (ii A. D.).
Commenting on the closely related Gospel of John, D.A. Carson (Exegetical Fallacies) brings up studies of agapē's meaning in other Greek literature.
they argue that translators of the Septuagint and New Testament writers have invested ἀγαπάω (agapaō, to love) and ἀγάπη (agapē, love) with special meaning to provide an adequate expression by which to talk about the love of God; and only this accounts for the word's rapid rise to prominence in our literature. But this argument has been overturned by the diachronic study of Robert Joly, who presents convincing evidence that ἀγαπάω (agapaō) was coming into prominence throughout Greek literature from the fourth century B.C. on, and was not restricted to biblical literature. This development was fostered by a number of changes in the language (linguists call them structural changes) in which ἀγαπάω (agapaō) was becoming one of the standard verbs for "to love" because φιλέω (phileō) had acquired the meaning to kiss as part of its semantic range.
In other words, the Greek in the new testament is not isolated from the general vocabulary of Koine Greek. There is an older thought that the new testament's Greek was a unique sort of 'Holy Spirit Greek', but this outdated idea was roundly disproven when the new testament was compared to contemporary Greek literature (both books and personal letters). Likewise, language is always evolving. Even a decade or two can drastically change how a word is used. This can make it difficult for us to compare how agapē may have been used in the Septuagint in previous centuries (especially since the Septuagint was not the product of a single set of translators), or in Greek literature in the following centuries.
In the time of 1 John, agapē seems to encapsulate a general, caring affection for others. For the author to say 'God is agapē' is to make a statement that God cares and is concerned for the well-being of his creation (specifically, humanity and the individuals within, in this context).
The word the author of 1 John uses is ἀγάπη. However, as to the meaning of that word, you will not find it accurately described in a Koine dictionary. The author of 1 John, in the same chapter teaches by example what he means by God being love:
NASB 1 John 4:10 In this is love, not that we loved God, but that He loved us and sent His Son to be the propitiation for our sins.
I believe this passage is intended to give the meaning of "love" in picture form.
Paul also despairs of explaining the love of God in Christ and instead prays that God would reveal it supernaturally. He specifically says that it "surpasses our ability to know it":
Eph 3:14 For this cause I bow my knees unto the Father of our Lord Jesus Christ, Eph 3:15 Of whom the whole family in heaven and earth is named, Eph 3:16 That he would grant you, according to the riches of his glory, to be strengthened with might by his Spirit in the inner man; Eph 3:17 That Christ may dwell in your hearts by faith; that ye, being rooted and grounded in love, Eph 3:18 May be able to comprehend with all saints what is the breadth, and length, and depth, and height; Eph 3:19 And to know the love of Christ, which passeth knowledge, that ye might be filled with all the fulness of God.
The lexicons on the hand describe it as it is used in the "world" as "general affection" and the like.