The English name of this card contains a slight mistranslation: being a simple reversal of "Testudo erat Numen," it does not actually say "The God That Will Be Called Turtle," but "The God That Once Was Called Turtle." "Numen erit Testudo" would be a more accurate translation.
This may perhaps be after the turtle has ascended to godhood from being a turtle. If this card has worshipers, they elevated him from "turtle" to "god", then called him "turtle" again after the destruction of the forgotten temple.
Only the German version of this card has a non-Latin name written "Göttlich war die Schildkröte". When translated into English, it would be "Divine was the turtle ".