Several older commentators tried to note Paul being fairly "sarcastic" here, as in: "Okay, you want a difference between the Jews and Gentiles? Fine: the Jews are justified by faith and the Gentiles through faith. Is that a good enough difference for you?" (See Calvin for this interpretation)
Just as many found the difference to lie chiefly in the use of the article not being present in the first case, but present in the second case (see some examples below)
Just as many again found no substantial difference at all (for example, Gill).
Alford finds not a contrasting difference, but different meanings to express how one and the same were justified:
"Too much stress need not be laid upon the difference of the two prepositions. The omission of the article in ek pist. and its expression in dia tes pist. are natural enough: the former expresses the ground of justification, generally taken, ek pist.; the latter the means whereby man lays hold on justification, dia tes pist., by his faith. The former is the objective ground, the latter the subjective medium."
A different view from Vaughan:
The two terms are equivalent and convertible, but they differ in form. The one is (1.) faith, the act of believing; the other is (2.) the faith, the object of belief, that is, the gospel. For an illustration of the distinction, see Gal. iii. 22-26.
Moo dismisses all such interpretations and finds simply a stylistic variation (and I don't have it in front of me, but I believe this was Cranfield's judgment as well? [Correct me if I'm wrong, someone]). So Meyer:
"The interchange of pisteos and tes pisteos in which the qualitative expression advances to the concrete with the article is also without special design, as similar accidental interchanges often occur in parallel clauses."
Olshausen again differs from all the above:
"[They] do not stand parallel to one another as designations of source and cause....Ek pisteos has a special reference to the Jews, who supposed that they were partakers of divine grace, not as believers, but simply as children of Abraham after the flesh."
Both Bengel and Godet follow similar trains of thought in their expositions.
Bottom line, there are almost as many interpretations of the difference in expression as there are able commentators to interpret it, and I don't think any could be argued as "more in keeping with orthodoxy" than the others, though some are indeed more exegetically sound.
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What is important to note is that both expressions are true for both Jews and Gentiles. There is no substantial or important difference between saying "through faith" and "by faith." Paul uses both expressions repeatedly. The question is simply one of why did he express them differently together in this one case? I'm inclined to put absolutely no contrast in Paul's mind here, but he was simply varying the expression (although I do find Calvin's interpretation quite nice).