...lohnt den Menschen(=Dativ) Göttergunst(=Nominativ).
This phrase does not contain an Akkusativ of "Menschen" but a Dativ, as can be seen, e.g., in the corresponding Wiktionary article. So the verb in question is "jemandem(=Dat.) lohnen". The Duden entry for "lohnen" has the following explanations:
1a: in ideeller oder materieller Hinsicht von Nutzen sein (i.e. to be useful)
1b: aufzuwendende Mühe oder Kosten rechtfertigen (i.e. to be worth the effort)
2: eine gute Tat, ein gutes Verhalten [mit etwas Gutem] vergelten (i.e. to reward for something)
There are some grammatical notes in the Duden entry:
For 1a the structure is "etwas(=Nominativ) lohnt sich(=reflexive pronoun)".
For 1b the structure is "etwas(=Nominativ) lohnt eine(r) Sache(=Genitiv or Akkusativ)."
For 2 the structure is "etwas(=Nominativ) lohnt jemandem(=Dativ) etwas(=Akkusativ)."
The first option (1a) cannot be meant in your example because, according to the Duden entry, it should be used with "sich" which is not present in your phrase.
The second option (1b) is not a possibility for your sentence because there is neither an Akkusativ nor a Genitiv.
The third option (2) fits your phrase, although there is no Akkusativ:
If love and strength combine, then the gods' favour(=Nominativ) will reward the human beings(=Dativ).
EDIT: I have just realised that "den Menschen" is both Akkusativ Singular and Dativ Plural. In my opinion the Akkusativ Singular interpretation is not possible because in each meaning (1a, 1b, 2) the person for whom the Göttergunst is useful/worth the effort/rewarding must be in Dativ case. So only the Dativ Plural interpretation is possible.