The considerations here are much the same as those I discussed in a previous answer. I have attempted to develop those ideas and tailor it to the passage in question.
[I]s it incorrect to read this clause as "love is God"?
Yes, it is.
In Greek, the subject of a clause can generally be identified as the substantive in the nominative case. However, in Greek (as in English), some sentences have two nominatives; this is known as a predicate nominative construction. It occurs when the verbal idea is "equative" rather than active. In 1 John 4:8b, we have:
Noun (nom): ὁ θεὸς = God
Noun (nom): ἀγάπη = love
Verb (equative): ἐστίν = is
Despite the term "equative", the two nouns have different syntactic and semantic roles. We call one the subject (S) and the other predicate nominative (PN). In English declarative clauses, these can be distinguished because S comes before the linking verb, whereas PN follows it. Greek, however, does not adhere to any particular word order. The question is: how to tell the difference between S and PN?
This problem has long occupied New Testament grammarians. Modern treatment of the problem seems to have begun with Eugene Van Ness Goetchius's 1966 formulation. By analyzing patterns in New Testament grammar, he found that S could be identified if either substantive was either:
- a proper name;
- an articular noun;
- a pronoun;
- mentioned in the immediately preceding context; or,
- if both are equally definite, the one with the narrower reference is S.
He was later criticized, particularly by Lane C. McGaughy, both for mixing syntactic and semantic categories and for potential ambiguity in application (what happens if two of the "rules" yield conflicting conclusions?). The more recent formulations (by McGaughy himself, re-iterated by Stanley Porter, and later by Daniel Wallace), are really not much different than the syntactic rules identified above (i.e. the first three).
- If either substantive is a demonstrative or relative pronoun, it is S.
- If either substantive has the article, it is S.
- If both have the article, the first in order is S.1
We can apply these criteria to 1 John 4:8: ὁ θεὸς (=God) has the article and is therefore the subject.2
The implications of distinguishing S and PN are similar in Greek and English. Most often, this frames a subset proposition. S is the narrower category (the hyponym); PN is the broader category that subsumes it (the superordinate). In this case, God is a specific individual within the broader category of love.3
1. Daniel Wallace also includes proper names in the list of S identifiers. It is arguable whether θεὸς should be considered a proper name in the New Testament; I think most would say that it should not. If it were proper, this only supports our conclusion.
2. Augustine's homily notwithstanding (see note 2365). I can't speak to the appropriateness of the Latin formulation.
3. Of course, such propositions do not normally mix individuals and abstract concepts. This is intentionally, I think, jarring and characteristically Johannine (cf. 1:5, "God is light" and 4:24 "God is spirit"). Fortunately, we are not left on our own to operationalize this: "And this is how God's love was revealed among us: that God sent his only son into the world..." (4:9).
E. V. N. Goetchius, The Language of the New Testament. New York: Scribner’s, 1965.
L. C. McGaughy, Toward a Descriptive Analysis of Εἶναι as a Linking Verb in New Testament Greek. Missoula, Mont.: Society of Biblical Literature, 1972.
Stanley E. Porter, Idioms of the Greek New Testament. Sheffield: JSOT, 1992.
Daniel B. Wallace, Greek Grammar Beyond the Basics: an Exegetical Syntax of the New Testament. Grand Rapids: Zondervan, 1996, 40-43.