Ya estaba de Dios, “it was already God’s,” means that something could not be avoided. The Diccionario del español de México gives the forms ya estaría and ya estaba de Dios, marks them as popular and explains them as “Ser ya algo inevitable o fatal” (for something to be inevitable or fated).
Its example: “Ya estaría de Dios que me asaltaran” (It must have been God’s will that I was robbed).
In his essay “Teoría mexicana de lo adverso” (Mexican Theory of the Adverse), Monsignor Joaquín Antonio Peñalosa places ya estaría de Dios beside ese era mi destino, “that was my destiny,” and ya me tocaba, “my turn had come.”