I am a learner also, and I prefer to write in English because this is a complicated topic, so please indulge.
Clauses starting with, "if..." can be either in the indicative mood or in the subjunctive mood. In the statement above, when "Dios" spoke (imperfect past), even as a supposition (if), there is neither doubt, nor is it counterfactual. Therefore, the subjunctive case here is not appropriate.
One can see a case where both in English and Spanish, we use the subjunctive tense. This is in the form of an "irrealis" :
If I had a million dollars, I would buy myself a Ferrari. (past subjunctive)
Si tuviera un millón de dólares, me compraría un Ferrari. (past subjunctive)
Here, the past subjunctive is used because the scenario is not real. It is a supposition only, and has virtually no chance of actually being true. But there can be some difference in other "if..." tense combinations that can differ between English and Spanish.