ἦν δὲ τὸ α ἐπιχείρημα τὸ ἀπὸ τῆς θείας γνώσεως, ὅπερ κατὰ τὸν Ἰάμβλιχον ἐπελύσαντο.
The first argument was by means of divine knowledge, which they explained according to Iamblichus.
I do not understand the second τὸ. It looks as if it were not the predicate but an apposition to τὸ α ἐπιχείρημα. But then where is the predicate? Maybe, translating: the first argument was the one by means of divine knowledge would be better.
When he talks about the second argument, he omits τὸ with the copula: τὸ δὲ β ἀπὸ τῆς ἀκολυθἰας τῶν λόγων …
Maybe ἦν of existence?
There was the first argument from divine knowledge, which they…
Perhaps he wrote it as an afterthought:
There was the first argument, the one from divine knowledge, which they…
When he gets to the second argument, he is distinguishing it from the first, rather then uniquely specifying it.
τὸ ἀπὸ τῆς θείας γνώσεως is predicate, as your translation suggests
It’s pretty jarring, if so. Can you give examples of something similar?